If you are AT&T customer and want to block other phone number, you can use some AT&T service to block it. These are some feature that AT&T provide to their customers to block phone number using Caller ID.
Using a display unit,AT&T Caller ID allows you to see the telephone number of the caller before you answer the phone. Even if the caller's number is unlisted, the number will be displayed — along with the date and time of the call.1 The subscriber must supply Caller ID equipment. To subscribe, please contact AT&T Customer Service.
Caller ID With Name
Using a display unit, AT&T Caller ID with Name allows you to see the telephone number and name of the caller before you answer the phone. Even if the caller's name and number are unlisted, this information will be displayed — along with the date and time of the call.2 The subscriber must supply Caller ID equipment. To subscribe, please contact AT&T Customer Service.
Caller ID Blocking
AT&T Caller ID Blocking provides you with two privacy options for your outgoing calls: Per Call or Per Line. When you use these options, your name and number will not be displayed to the called party.
To activate Caller ID Blocking Per Call:
1. Listen for a dial tone.
2. Press *67 and wait for a second dial tone.
3. Dial the outgoing phone number.
The receiver of this call will not see your phone number or name for this call only. If you want to block your information from being transmitted on the next call, you must repeat steps 1 to 3 above.
To deactivate Caller ID Blocking Per Call:
• Automatically deactivated when you hang up from the original call.
To activate Caller ID Blocking Per Line:
• Customer must contact AT&T to subscribe to this service.
• This service lets subscribers block the display of their name and phone number on all outgoing calls on a line designated by the subscriber.
To deactivate Caller ID Blocking Per Line on a Per-Call Basis:
1. Listen for dial tone and press *82.
2. Complete the call as usual and subscriber's Caller ID information will be sent.
Anonymous Call Rejection
AT&T Anonymous Call Rejection automatically rejects calls from people who intentionally block delivery of their calling party name and/or number. When purchasing Caller ID service, this feature is automatically placed on your business line.
• To activate Anonymous Call Rejection, press *77 (or dial 1177 for rotary phones)
• To deactivate Anonymous Call Rejection, press *87 (or dial 1187 for rotary phones)
AT&T Distinctive Ring allows you to have two telephone numbers on a single analog line. The distinctive ring number will have a unique ringing pattern, which allows you to easily identify calls to each number. To activate or deactivate Distinctive Ring, please contact AT&T Customer Service.
Selective Call Rejection
AT&T Selective Call Rejection allows you to selectively reject incoming calls from up to 12 different telephone numbers, which you identify.
To activate Selective Call Rejection:
1. Listen for a dial tone and press *60.
2. Listen for the announcement of current "on/off" status.
3. Listen for announcements outlining the available options.
4. Press 3 to activate your updates.
Follow instructions to add, remove, delete or review entries (step 3). This feature will not activate without at least one entry.
To deactivate Selective Call Rejection:
1. Listen for a dial tone and press *80.
2. Listen for instructions outlining available options.
3. Press 3 to deactivate the feature.
Source : ATT.com