Phone calls from unrecognized numbers are frustrating enough, but a call from a “blocked” or “private” number feels almost ominous. After all, why would a legitimate caller need to hide their number behind a restricted phone call?

What is a restricted phone call?

Every telecommunications provider in the U.S. offers caller ID services as an automatic feature, and virtually all smartphones and landline phones have built-in caller ID displays. As a result, people now rely on caller ID information on their phones—and it can be uncomfortable when that information is blocked or not available.

All telecom providers allow customers to temporarily or permanently disable the caller ID feature. Depending on the carrier and type of phone you have, it may be as simple as enabling a feature on a smartphone app or entering a prefix such as *67 before you make a call. When caller ID is disabled, the person receiving the call only sees a word such as “blocked,” restricted,” “private” or “unknown” on the caller ID display.

Restricted calls are fast becoming a popular tactic with scammers placing robocalls.

“The [telecom] carriers started to identify the bad guys. Call-blocking apps started to scale up and gain popularity,” Alex Quilici, CEO of YouMail, recently told NBC News. As a result, the scammers have to try new tricks, such as disabling caller ID, to reach their targets.

It’s not just telemarketers and scammers using restricted calls. Bill collectors often employ the tactic to disguise their identity, as do harmless prank callers.

Worst of all, though, stalkers and other criminal types can use caller ID blocking to harass their victims anonymously. The Federal Communications Act, as well as laws enacted at the state level, exist to protect you from calls that are repeated or continuous, lewd or threatening, or creepily anonymous. This is classified as telephone harassment.

How can I block restricted phone calls?

The good news is, it’s easy to block restricted phone calls from both landline and cellular phones. Telecommunications providers have call-blocking features, as do most mobile phones. The steps to enable these features may be slightly different, depending on your carrier and specific phone model.

How to block restricted calls on a landline

If you have caller ID enabled on your landline, you’ll simply need to pick up the phone, dial *77 and wait for the confirmation message. Going forward, when someone who has blocked caller ID calls your number, an automated message plays prompting the caller to unblock caller ID and try the call again.

You can disable the feature by dialing *87 and waiting for the confirmation message.

How to block restricted phone numbers on a cell phone

Each cell phone carrier has different requirements and instructions for enabling anonymous or private call blocking. For example, you’ll need a Smart Family plan to block restricted phone calls with Verizon Wireless. AT&T offers anonymous call rejection as an optional feature for wireless customers. Depending on your carrier and plan, there may be an extra monthly fee for the service.

T-Mobile and Sprint currently do not offer blocking for restricted phone calls, but you may have options to block them directly from your mobile phone, depending on the model.

Other ways to handle restricted phone calls

If you want more information about your anonymous callers, especially if you believe they are harassing you by phone, you may have other options:

How to make a restricted call?

What do you do when you want to make a restricted phone call yourself? Perhaps you had your car repaired, but four days and $500 later the problem still exists—and now the shop isn’t answering your calls. Disabling caller ID may be the only way to get your complaint resolved.

If you’re calling from your house phone, you can *place a restricted call just by dialing 67 before the number you’re calling. The *67 feature also works on most mobile phones to temporarily block your caller ID. The downside is you can’t enter *67 on your keypad and then select a number from your contact list. So you’ll need to have the person’s number handy when ready to place the call.

How to make restricted phone calls on an iPhone

There are two options to block your ID on a mobile phone: Block caller ID for specific people in your contact list or block your number on every call you make.

To block caller ID for specific individuals:

Now you can call that person normally from your contacts or favorites list without manually entering *67 each time you call.

To block your caller ID for everyone, you may be able to toggle the option right from your phone settings if you have AT&T or T-Mobile mobile service:

If that function isn’t available on your iPhone or you have other carriers such as Verizon or Sprint, go to the carrier’s website or mobile app and look for the “services” or “devices” option. Then follow the instructions to make restricted phone calls.

Once you turn off your caller ID on an iPhone, you can enable it on a call-by-call basis by tapping *82 before the contact’s phone number. This may be useful if someone you want to call blocks or rejects restricted phone calls and you need to reveal your identity to get through.

How to make restricted phone calls on an Android phone

To turn off your caller ID for specific calls, tap *67 before you enter the phone number. You can also block your ID for specific contacts by adding *67 in front of their phone number on their contact page.

To disable caller ID for all outgoing calls, go to your home screen and tap the Settings app. Select “Additional settings” and then click “Caller ID.” Choose “Hide number” from the available options to turn on the feature.

Like iPhones, once you turn off caller ID on Google Android phones, you can enable it on a call-by-call basis by entering *82 in front of the phone number.