AT&T 7406BIS - Manual (Page 24)

Automatic Callback
To automatically place 1 Press [ Auto Callback ] before or during call attempt
another call to an [dial or confirmation tone]
extension that is or dial Automatic Callback code before call attempt
frequently busy [dial tone]
●
If you have an [
Auto Callback
] button, green light goes on
until callback is completed or canceled
2 Hang up (within 7 seconds) or press [ Drop ]
●
You will receive a 3-burst priority ring when both
your voice terminal and the called extension are idle
3 Lift handset when you hear priority ring
[ringback tone]
●
A call is automatically placed to called extension,
which receives regular ringing
Note: Only one Automatic Callback call at a time can
be placed, and Automatic Callback is canceled after 20
to 40 minutes if the callback call is unanswered.