AT&T U-Verse Do Not Disturb
This feature will give callers a busy signal until you turn it off.

To activate dial:

*78#

To de-activate dial:

*79#


Helpful Hints:

1. This feature is only available to AT&T U-Verse customers (although other companies may have similar features under a different name). 
2. On a traditional landline simply leaving your phone off the hook will have the same effect.