Call blocking, also known as call block, call screening, or call rejection, allows a telephone subscriber to block incoming calls from specific telephone numbers. This feature may require an additional payment to the subscriber's telephone company or a third-party.
Unwanted calls to landlines may be blocked through a number of methods. Some landline phones have built-in call blocking facilities. External call blockers are sold as telephone accessories which plug into existing phones.
Call blockers and related services recently received attention in 2016 from publications including Which? and Consumer Reports in the United Kingdom and United States respectively.  Such devices and services enable the user to block a call as it is in progress or alternatively block the number after the call is made. These devices rely on caller ID information and thus a phone blocker requires a caller ID service active on the line for blocking to function.
It may also be possible to use computer software, in conjunction with the caller ID information from the phone company and a caller ID enabled phone modem to block an incoming call.
Treatment of blocked calls may include:
- Sending caller to voicemail
- Sending caller to busy signal
- Sending caller to "Number No Longer in Service"
- Sending caller to "Keep Ringing"
Spam calls to mobile numbers can be blocked by an in-built call blocking feature on your smartphone or special call blocking applications that provide an up-to-date spam numbers database and block all suspicious calls by category.
- Leedham, Robert. "Call blocking devices – everything you need to know". which.co.uk. Which?. Retrieved 13 January 2016. CS1 maint: discouraged parameter (link)
- "Rage against robocalls". gametechia.com. game techia. Retrieved 13 January 2020. CS1 maint: discouraged parameter (link)
- "How to block phone calls on your Android phone". digitaltrends.com. Digital Trends. Retrieved 13 January 2016. CS1 maint: discouraged parameter (link)