AT&T System 25 - Manual (Page 17)

Call Waiting
If all of your System Access buttons are busy, you may be
notified (through your handset) that a call is waiting. If you
hear one short tone, it is an inside call waiting; two short tones
indicate you have an outside call waiting.
If you have a System Access-Outgoing button on
your telephone, you can put your calls on hold and answer the
call waiting. If you do not have this button, you must end one
of the existing calls in order to answer the call waiting.
Answering - Using SYS ACC-O
1 Press
2 Press .
3 Press
Answering - Ending a Current Call
1 End your conversation and hang up.
2 When the telephone rings, lift the handset.
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