Getting calls from unknown or blocked numbers can be annoying and even worrying. You may want to know who is calling you for many reasons – to avoid unwanted calls, stop potential harassment, or identify legitimate calls you may want to answer. While caller ID provides the number, it doesn’t always provide the name or location.
Fortunately, there are several effective ways to find out who an unknown caller is.
Is it Possible to Find Out an Unknown Caller Number
Yes, it is possible to find out who an unknown or blocked caller is even if they have hidden their number. Here are some effective ways to identify an unknown caller:
- Use a reverse phone number lookup service like Whitepages or TruePeopleSearch. They maintain databases of phone number records that allow you to enter a number and see the name and address associated with it.
- Caller ID apps like Truecaller and Hiya cross-reference unknown numbers against user-submitted databases and can provide a name and location of the caller.
- Dialing *69 or *57 after receiving an unknown call will perform a callback trace on many landline and mobile networks, allowing you to later request the traced number from your phone carrier.
- Your phone carrier may offer paid services to provide caller name and location details that are unavailable through free options.
- If it’s a repeat unknown caller, try answering and politely asking for their name and reason for calling.
- For robocall scams, report the number to the FTC who can subpoena records to legally identify and trace spam callers.
- Paid services like Intelius provide guaranteed reverse phone lookups for a fee by searching proprietary data sources.
So while it may take some detective work, there are ways to eventually find out the identity of even the most persistent unknown nuisance callers contacting your phone. With the right approach, you can uncover who is trying to reach you and why.
How does Unknown Caller Number Works
Here’s an overview of how unknown and blocked caller numbers work:
- Callers have the option to block their number from being shown on your caller ID display by dialing *67 before making a call on most networks. This hides their identity.
- When a blocked call reaches your phone, it typically displays as “Private”, “Anonymous”, “Unknown”, or “Blocked”. On iPhones, it may simply say “No Caller ID”.
- If you answer an unknown call, the caller has the choice to identify themselves or remain anonymous. You won’t be able to automatically see details about them.
- Telemarketers and scammers often use blocked calls to hide their real numbers and identities. This makes it harder to avoid or report them.
- Legitimate callers may block their numbers to protect privacy. But repeated anonymous calls can be a red flag for harassment or illegal robocalling.
- On the technical side, blocking works by stopping the transmission of Caller ID details that normally accompany calls between networks.
- Even if the number is blocked, your carrier still receives info on where the call originated for billing and routing purposes.
- That’s why you can use tracer services like *69 or *57 to identify some blocked callers through your phone company.
- In summary, anonymous calling allows privacy but is easy to abuse. Tracing options and common sense can help identify truly troublesome unknown callers.
So while unknown calls may seem completely untraceable at first, there are ways to eventually uncover the source with some time and effort.
How to Find Out an Unknown Caller Number (Easy Methods)
Getting calls from unknown numbers can be frustrating and disruptive. But there are several ways to uncover who is calling you from a blocked or unidentified number. Here are 6 effective methods you can use:
Method 1: Use *69 to Trace Recent Unknown Calls
The *69 code is a free feature offered by many phone carriers that allows you to identify some unknown callers. To use it:
- Dial *69 on your phone’s keypad immediately after receiving an unknown call.
- An automated system will provide the name and number associated with the last incoming call to your number.
- Write down the details provided so you have a record of the caller ID.
- Note this only works on landlines and some mobile networks. Results vary based on carrier offerings.
Method 2: Trace Unknown Calls with *57
Similar to *69, the *57 code allows tracing on some landline and mobile networks. To use:
- Dial *57 immediately after receiving an unknown call.
- This marks the calling number in the carrier’s system.
- You can then call the provider to request the details on the traced number.
- Keep in mind mobile support depends on carrier and landlines may require a monthly subscription.
Method 3: Check with Your Phone Carrier
Your phone provider may offer options to view, trace or block unknown calls. Contact them to ask about:
- Enhanced caller ID services that identify unknown mobile or landline numbers.
- Options to “unmask” blocked numbers so you can view the details.
- Instructions for tracing nuisance calls through codes like *57.
- Ability to block or filter calls from private, unknown, or anonymous numbers.
- Submitting unknown numbers to have them blocked by the carrier across its network.
Method 4: Use Third-Party Caller ID Apps
Apps like Truecaller, Hiya, WhosCall provide enhanced caller ID services by cross-referencing unknown numbers against user-generated and crowdsourced databases. Key features include:
- View names and locations associated with unidentified numbers
- Option to report unsolicited calls as spam to improve app blocking
- Reverse phone number lookups by entering a number to search for details
- Blocking of suspected robocalls and telemarketers based on screening algorithms
Method 5: Report Scam Calls to the FTC
If an unknown number is engaging in obvious scam calls:
- Note all identifying details about the call – exact number, content, caller voice, company names mentioned etc.
- Report it immediately to the Federal Trade Commission online or by calling 1-888-382-1222.
- This helps authorities identify larger illegal robocall operations and take action.
Method 6: Use Phone Settings to Silence Unknown Callers
Both iPhone and Android provide call silencing options to limit interruptions from unknown callers:
- iPhone – Enable Silence Unknown Callers under Phone Settings > Siri & Search
- Android – Turn on Block Unknown Callers under Settings > Blocked Numbers
This automatically sends unknown calls straight to voicemail. Legitimate callers can leave a message identifying themselves while robocalls hang up.
Getting repeated calls from an unidentified number can be unnerving. With persistence across these tracing methods, you can uncover who is calling you and take steps to end disruptive unknown calls.
Alternative Methods to Find Out an Unknown Caller Number
there are several effective ways to find out who an unknown caller is.
Method 1: Use Reverse Phone Lookup Services
One of the quickest ways to identify an unknown number is to use a reverse phone lookup service. These services maintain extensive databases of phone number records that allow you to enter a phone number and find out the owner’s name and location.
Here are some top reverse lookup services to consider:
Whitepages is one of the largest phone directory services that offers a free reverse phone lookup. Simply enter the phone number on their website or mobile app and you can view the name and address associated with it, if available. They compile data from public sources as well as contributions from users.
Truecaller is a popular app for Android and iOS devices that has an extensive crowd-sourced database of contact details and spam markers. The free version allows you to do a limited number of lookups per month while the premium version has unlimited lookups.
ZoomInfo is targeted more at businesses but they allow some free reverse lookups per month on their website. They gather data from a variety of public and proprietary sources to provide detailed consumer and business contact information.
SpyDialer offers a free reverse phone lookup service on their website. They provide basic name and address details for cell phones and landlines. Additional details like email addresses, social profiles, and photos require a paid account.
The major advantage of using a reverse lookup service is the ability to instantly identify unknown callers. However, they will not work for numbers that are unlisted, landlines with no public directory listing, or VoIP lines. Results also vary between services depending on their data sources.
Method 2: Utilize Your Phone Carrier Resources
Your phone carrier may offer options to identify unknown callers through free or paid services.
1. Caller ID
Many carriers offer a caller ID function on mobile phones and landlines so the incoming number is displayed. Activating this can immediately show you the number and where it is registered. However, this doesn’t provide the name if the number is not already in your contacts.
2. Channels for Reporting Spam Calls
Major carriers allow you to report spam phone numbers by either marking it on your phone or submitting it through their website or mobile app. This helps block the number from calling you and adds it to their databases.
3. Paid Services
Carriers may offer premium services or apps that provide enhanced caller ID features for a fee, such as names for unknown numbers and spam blocking. For example, AT&T offers the Call Protect app while T-Mobile has the Name ID app.
Checking with your phone carrier first can sometimes provide quick and simple options specific to your service. However, the capabilities may be limited compared to third-party reverse lookup services.
Method 3: Look Up the Number Online
There are several free websites and forums online that allow you to manually search for identifying details on a phone number:
This community-based site allows you to search for a number to see any details submitted by other users. You can also contribute any information you have on the number.
800Notes has a large collection of user comments on 800 numbers, which can provide info on businesses associated with toll-free numbers.
3. Google Search
Try searching the phone number on Google directly. You may find forum posts or business listings with details on the number.
4. Facebook Search
Enter the phone number on Facebook’s search bar to see if it is associated with any user profiles. This works for identifying individuals but not businesses.
The main drawback with these manual search methods is that they require some effort and may not provide complete information. The results can also be hit-or-miss based on user contributions.
Method 4: Use Apps to Identify Spam Calls
There are several call screening apps that can automatically identify spam phone numbers:
RoboKiller has an extensive spam database that it uses to block and identify spam calls. It also allows you to create custom block lists.
Nomorobo screens and blocks robocalls as well as alerting you to potential spam calls so you can choose to block them.
Hiya identifies unknown callers and lets you know if they are suspected spam. It also gives you the option to block or report numbers.
The advantage with call screening apps is that they tap into constantly updated spam databases. The downside is that they may incorrectly flag legitimate calls as spam.
Method 5: Perform a SIM Card Search
If phone number lookups don’t provide details, you can try searching the SIM card associated with the phone number through sites like FreeCarrierLookup.com and NumberingPlans.com.
To use these sites:
- Enter the phone number without country/area code.
- Select the country and carrier associated with the number.
- The search will provide details on the mobile carrier and location of the SIM card.
This technique can give you the service provider and location which provides some additional clues on the source. However, it does not reveal the identity of the caller.
Method 6: Use Social Media to Identify the Caller
If you regularly get calls from the same unknown number, social media can provide additional clues on the caller’s identity based on how they interact with you.
Some ways to leverage social media include:
- Post about the unknown number on your social media accounts and ask if anyone recognizes it. Friends from the same area code may identify local businesses or people associated with the number.
- Check social media profiles of businesses or people in the area code for any mentions of the phone number on their profiles or posts.
- Look up social media profiles using the phone number as the search term to uncover associated accounts you can investigate for clues.
- Use your own social media accounts to indirectly engage the caller and gauge reactions that may reveal their identity based on how or if they respond.
This approach takes more effort but can uncover clues based on the caller’s digital footprint and reaction to your engagement. If the caller contacts you repeatedly, you may be able to gather helpful info to eventually identify them.
Method 7: Use Paid Identification Services
If you need to urgently identify a caller and free options have not worked, there are paid services that provide guaranteed caller identification:
- TrapCall: Unmasks blocked calls so you can view the hidden number and identify the caller. Plans start at $8.99 per month.
- PhoneLookup: Provides unlimited reverse phone lookups starting at $9.99 per month, including identifying cell phones.
- Intelius: Offers monthly unlimited plans for $4.99+ per month, giving you access to comprehensive background checks and phone lookups.
While pricier than free services, paid identification services employ dedicated research teams and access proprietary data sources to identify almost any caller. This can save you time and provide peace of mind.
Method 8: Consider Going Directly to the Source
If you cannot identify the caller through lookups and research, it may be most effective to directly ask the source:
- Simply answer the call: Politely ask who is calling and request details like their name, company, and reason for calling. If it is a wrong number or prank call, ask them to stop contacting you.
- Text the number: Send a text to the unknown number requesting details on their identity. Include your name and indicate you missed a call and want to call them back once identified.
- Call back: If anxious about answering directly, call back to hear if there is a voicemail greeting identifying the person. You can then explain you received a call and hang up.
- Setup voicemail: Update your voicemail to say you do not accept unidentified calls and request callers leave their details. Check frequently for any messages.
- *Use 67: Dial *67 before the number to block your identity. Ask for their name and state you are returning an unknown call. Hang up if uncooperative.
Going directly to the source can be effective for eventually determining the caller’s identity in a harmless manner. For persistent unknown calls, it may be your best solution.
How to Blocking Unwanted Calls
1. For iPhone
No one doesn’t expect unknown calls and doesn’t want to talk. So, those who have iPhone, they can block all kinds of unknown calls automatically that aren’t in their contact list. There is features in iPhone named ‘Silence Unknown Callers’, the iPhone users can use it if they get many unknown calls.
To active this features, you have to follow some steps are given below:
- At first, you have to go to your phone ‘setting’, and you have to scroll down and you will find the option named ‘Call Silencing and Blocked Contacts’.
- Now, there will be ‘Silence Unknown Callers’, and you have to switch on of this option.
- If you can do these steps perfectly, the caller will receive a message like that you are disconnected from call.
2. For Android
Android phones also have this feature. Follow the following steps:
- First of all, Android users have to open ‘The Dialer’, and there are visible three vertical dots on the right side of the top screen.
- You have to tap on it, and you will find ‘Settings ‘.
- If you tap on it, there will be visible ‘Block Numbers ‘ or ‘Call blocking’.
- Now, turn on the ‘Block anonymous calls’.
- If you want to block particular numbers, you should tap ‘Block Numbers ‘ and write the number or add.
How to Check Unknown Number Details Online
Here are some effective ways to check details on an unknown phone number online:
- Reverse Phone Lookup Sites: Use sites like Whitepages, TruePeopleSearch, or NumLookup. Enter the unknown number to search their databases of phone directories, public records, and user submissions. This can provide a name, address, carrier details, and more.
- Carrier Lookup Sites: Websites like FreeCarrierLookup.com and NumberingPlans.com allow searching a number to identify the associated cell phone carrier. This reveals where the phone is registered at minimum.
- Facebook Search: Copy and paste the unknown number into Facebook’s search bar to see if any user profiles show up associated with that number.
- Google Search: Search the full phone number in Google to surface any relevant forum posts, comments, or profiles that may reveal details.
- 800notes: This community-based site aggregates user feedback on 800 numbers. Look up toll-free unknown numbers here.
- PhoneNumberLookup.org: Allows searching unknown numbers to see if other users have provided intel or comments on the source.
- Paid Background Check Sites: For guaranteed results, sites like Intelius and TruthFinder provide robust background checks on any number for a fee.
- FTC Complaint Assistant: Report scam robocalls from suspicious unknown numbers to aid tracing efforts.
With persistence across these various online options, you can uncover at least some details on most unknown callers contacting you.
How to Identify an Unknown Caller
Here are the most effective ways to identify an unknown caller:
- Use a reverse phone lookup service: Reverse phone lookup services like Whitepages and ZabaSearch allow you to enter a phone number and find out who it’s registered to. They search public records, phone listings, and directory databases to find a name and address.
- Try caller ID apps: Apps like Truecaller and Hiya identify unknown callers by cross-referencing the number against crowd-sourced databases of user-submitted info. Many provide name, location, and caller reputation.
- Dial *69 or *57 for last call return: Dialing *69 or *57 right after getting an unknown call traces it back to a revealed number on many landline and mobile phones. Check with your carrier on availability.
- Contact your phone carrier: Your carrier may offer enhanced caller ID or services to “unmask” blocked numbers for a fee. Ask about options to submit unknown numbers for blocking too.
- Search online and social media: Search the phone number on Google, Facebook, and community sites like 800notes.com to see if others have identifed the caller.
- Use call screening apps: Apps like RoboKiller and Nomorobo automatically block and label suspected robocalls so you can identify likely spammers.
- Pick up the call: If it’s a repeat unknown caller, answer the call and politely ask for their name and reason for calling to gather clues.
- Pay for identification services: Paid services like PhoneTray and Intelius guarantee identification by searching specialized databases. Useful when other methods fail.
With persistence across these different techniques, you can ultimately put a name to most anonymous callers that contact you repeatedly.
How to Trace Unknown Number: Easy Methods
Here are some effective methods to trace an unknown number that has called you:
Method 1: Use Call Return (*69)
- Dial *69 on your landline or mobile phone immediately after receiving a call from the unknown number.
- This initiates a callback trace to identify the last incoming call.
- When prompted, the automated system will state the phone number that just called you.
- Write down the number for future reference.
- Limitations: Only works on some landline and mobile networks depending on carrier support. Results can be inconsistent.
Method 2: Try Call Trace (*57)
- Dial *57 right after getting a call from the unknown number.
- This signals your carrier to flag the incoming call in their system.
- Contact your phone carrier within a few days and request they provide details on the traced call.
- May require a monthly subscription fee on landlines.
- Only certain mobile carriers offer this feature.
Method 3: Use a Reverse Phone Lookup
- Search the unknown number on sites like Whitepages, TruePeopleSearch, or NumLookup.
- These sites cross-reference billions of public records and phone listings to find a name.
- May provide name, address, age, and other details on the caller.
- Limitations: Doesn’t work for unlisted numbers, VoIP lines, or numbers with no public info.
Method 4: Install a Caller ID App
- Download apps like Truecaller, Hiya, or Mr. Number.
- These apps identify unknown callers by comparing the number against crowd-sourced spam databases.
- Provides caller name and location info right on your smartphone’s caller ID.
- Can automatically block likely spammers.
Method 5: Check with Your Phone Carrier
- Contact your phone carrier’s customer support.
- Ask if they offer enhanced caller ID or services to unmask blocked numbers.
- See if they allow submitting unknown numbers to have that caller blocked.
- Carriers have some unique options that aren’t available through third-party services.
Method 6: Report Potential Scams
- Note all details about calls from any suspicious numbers.
- Report them to agencies like the FTC and FCC for investigation.
- Authorities have ways to legally trace scam and spam numbers even if blocked or spoofed.
With persistence across these options, you can ultimately trace most unknown callers contacting you.
How to Find Unknown Number Name
Here are some effective methods to find the name associated with an unknown phone number that has called you:
- Use a reverse phone lookup service like Whitepages, ZabaSearch or TruthFinder. Enter the phone number on their site to search their databases of public records, phone directories and user submissions to find a name.
- Install a caller ID app like Truecaller, Mr. Number or Hiya on your smartphone. These apps identify caller names by cross-referencing the number against crowd-sourced databases maintained by app users.
- Check your wireless carrier’s website or call customer support to ask if they offer a name identification service for unknown numbers. Some carriers provide this for a fee.
- Use Facebook’s search bar to enter the phone number and see if it brings up a user profile associated with that number that provides a name.
- Try searching the phone number on Google followed by keywords like “phone” or “call” to surface any forums, listings or public posts with identifying details.
- Dial *69 after receiving a call from the unknown number to do a call return trace and get the name stated out loud by the automated system.
- Use paid services like Intelius and PeopleLooker that guarantee name and detailed background info on any number through extensive proprietary data sources.
- Ask the caller their name directly if it’s a repeat unknown caller. Pick up the call and politely ask who is calling and why to collect identifying details.
With patience and persistence using both free and paid services, you can eventually put a name to most persistent unknown numbers that call your phone.
How to Be an Unknown Caller
You can be an unknown caller by hiding your number. *67 is the code number, it will help you to hide or mask your caller ID information that you are using. If you want to hide your caller ID on every call, you have to dial *67 at first and then the number of the person you want to call.
You can also hide your caller ID permanently. you have to call your mobile or phone carrier and tell them to permanently block your mobile number from caller IDs. If once you do it, it cannot be changed and it’s permanent. If it is successfully activated, when you will call someone, your contact number will be displayed to them, it will be shown as “Private Number”.
How to Tell if it’s a Spam Number
Watch for these common red flags that indicate an unknown caller is likely a spammer or scammer:
- The call comes from an ambiguous number like 000-000-0000.
- They immediately launch into a sales pitch or warning without identifying themselves first.
- It’s an automated robocall recording.
- The number shows up as “Potential Spam” on your phone’s caller ID based on crowd-sourced databases.
- You get repeated calls from the same number, often at odd hours.
- The caller asks for personal information like your social security number.
- The number is associated with multiple complaints of spam calls when you do a reverse phone lookup.
How Do Callers Hide Their Phone Number
Callers have a few ways to hide or spoof their real number:
- Using *67 before dialing blocks their number from appearing on your caller ID
- Downloadable apps can allow number spoofing, often used for prank calls
- VoIP services give users options to mask their numbers
- Robocallers use auto-dialers that cycle through fake numbers
- Scammers actively spoof legitimate-looking numbers to appear local
How to Find Out Who an Unknown Caller Is
- Use reverse phone lookup services like Whitepages and Truecaller by entering the number to search available name and address records.
- Dial *69 or *57 immediately after a call to trace it back to a revealed number in many cases.
- Contact your phone carrier—they may offer enhanced caller ID or options to unmask blocked numbers for a fee.
- Install caller ID apps like Hiya that identify unknown numbers using crowd-sourced databases.
- Search the phone number online using sites like 800notes.com and phone-number-lookup.org to see if others have provided info on the caller.
- Pick up or return the call to coax identifying info out of the caller through conversation.
How to Block Unwanted Calls
Reduce unwanted calls using these blocking methods:
- Add specific numbers to your smartphone’s block list to automatically send to voicemail.
- Use the Do Not Disturb feature to silence all calls from anyone not in your contacts list.
- Contact your carrier to request blocking of individual nuisance numbers.
- Sign up for Nomorobo or apps that automatically block and screen robocalls.
- Register your number on the National Do Not Call Registry to legally stop sales calls.
How do I Find Out an Unknown Number
- Dial *69 on a landline or enabled mobile phone right after getting the unknown call to do a callback trace.
- Use a reverse phone lookup service by entering the number to search available name and location records.
- Install a caller ID app that can cross-reference the number against crowd-sourced spam databases.
- Ask your phone carrier if they can provide caller name details for unknown numbers.
Can you Unmask an Unknown Number
Yes, there are a few ways to reveal an unknown number:
- Dialing *57 on supported networks traces the number which you can then request from your carrier.
- Your phone carrier may offer a paid service to unmask blocked numbers.
- Apps like TrapCall let you directly unmask blocked calls on your smartphone.
- If law enforcement gets involved, they can subpoena the registered number from phone providers.
Can Star 67 be Traced
In most cases, no. Using *67 blocks your number from being shown on caller ID when making calls. However, some carriers can override this if requested by law enforcement during an investigation of illegal robocalling or telephone harassment.
What does *57 do
Dialing *57 on landlines and some mobile networks initiates a trace on the last incoming call. Your carrier can then provide details on the traced number at your request or subpoenaed by authorities. Like *69, its effectiveness depends on provider implementation.
When to Take Further Action
If an unknown caller is clearly harassing you or making threats, take these additional steps:
- File a report with the police so there is an official record in case the calls persist or escalate.
- Keep a log of call dates, times, content, and any identifiable info like gender, voice cues, number patterns. This supports investigation efforts.
- Contact your phone provider to block the number and submit any collected details. They may have additional blocking capabilities and info to aid authorities.
- Consider changing your number as a last resort if the caller continues to be unidentified and harassing. This is disruptive but may be necessary.
Your safety is the priority. Reach out for professional support if an unknown caller becomes dangerous or excessively threatening.
Key Takeaways on Identifying Unknown Callers
- Use reverse phone lookup services to quickly find name and location details associated with a phone number.
- Check with your phone carrier for built-in caller identification options and channels to report spam.
- Search the phone number online using websites and search engines to find user-provided info.
- Install call screening apps that identify and block spam calls and robocalls.
- Attempt to gather clues directly from the source by answering, calling back, texting or leaving voicemail messages.
- Turn to paid identification services for urgent situations requiring guaranteed results when other options fail.
- Involve the authorities if an unknown caller is engaged in obvious harassment, threats or illegal activity.
With persistence across these different methods, you can eventually identify most unknown callers to gain control over unwanted and disruptive phone contacts. Proactively taking steps to research and engage such calls sends the message that you cannot be easily harassed or intimidated over the phone.
All-time, an unknown call is not disturbing or annoying. Sometimes, it may be from your closed ones or important calls of you. If you find that it is totally unrecognised to you and repeatedly disturbs you and it goes harmful to you, you should take action from then. Following any of the options from above, you can easily punch in the anonymous number and also find the contact details of the person who called you.
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Frequently Asked Questions (FAQs)
An unknown number that is not familiar to you, or it is not in your list with or a blocked caller ID, this is an unknown caller.
Through TrapCall, you will be able to get the unknown caller’s information and number and also the blocked number if you get a call again from them. You can use this app on both iOS and Android devices. It reveals the number of anonymous calls in real-time. Once you find the number you can easily block it.
You can dial *57. It will help you to discover the identity of an unknown caller is a 57 call trace.
You can use *69, though it is generally used for blocking unknown callers. You can also use it to call back an unknown number. You have to dial *69 to automatically call back the last number that called you for landlines. For cell phones, you can dial #69 on mobile phones to return the call of the recent private numbers.
NumberGuru, is a free service and you will be able to find out quickly who is calling you by using it from your smartphones.
Use a reverse phone lookup service, caller ID app, or dial *69 or *57 to trace recent unknown calls. Contact your carrier for unmasking services or report spam callers to authorities to aid in identification.
Dialing *67 blocks your number from appearing on the receiver’s caller ID display. The call will show up as anonymous or unknown to the person you are calling.
Yes, police can work with carriers to override blocking and trace nuisance, threatening or harassing calls as part of an investigation. Carriers maintain records of all call origins.
Debt collectors may intentionally block their numbers to avoid screened calls. This unethical tactic forces recipients to answer anonymous calls. Ask immediately for validation if suspecting a collection call.
On iPhones, enable ‘Silence Unknown Callers’ under Settings > Phone to automatically send unknown calls to voicemail. You can also block specific numbers.
It’s not illegal, but repeatedly calling someone anonymously to harass or threaten them can result in criminal charges based on tracing by authorities.
Anonymously call back and request to be removed from their calling list. Or answer once and firmly insist they stop contacting you. Report excessive unknown calls to your carrier and consider blocking the number.