Call blocker, or selective call rejection, is a feature offered by most landline phone service providers. The feature allows you to selectively block certain callers from calling your phone. When a call is placed from a blocked number, a recording plays in the caller's earpiece, alerting the caller that you are not currently accepting calls. If you do not know the number of the caller, you can still block the number. The unknown caller must be the last person to have called in order to block the phone number.
Pick up your landline phone's handset and listen for the dial tone.
Dial "*-6-0" -- that's the asterisk or "star" button at the bottom of the keypad, along with "6" and "0" -- and listen for the call blocker system greeting. If you are dialing from a rotary phone, dial "1-1-6-0" to access the call blocker system.
Follow the system's prompts to block calls made by the phone that last called your phone. The prompts vary by phone service provider. Be sure to confirm your selection to block the last caller.
Wait for confirmation that the number was blocked, then hang up the phone.
If you just want to force unknown callers to display their caller information, you can add Anonymous Call Rejection to your phone service. When the feature is activated, unknown callers are informed they must unblock their caller information before the call will be allowed to go through. Contact your phone service provider to add the feature.