AT&T System 25 - Manual (Page 58)

Recall
If a System 25 telephone or outgoing line is busy when you
attempt to place a call, you can press
and hang up.
When the telephone or outgoing line becomes available, the
system will ring your telephone. For more information about
this feature, see the “Callback” section.
In addition, if your company uses certain Central Office features,
Check with your System Administrator for a description of
available features, access codes, and procedures.