AT&T SYSTEM 25 Manual page 29

Multiline
Answering by Holding Other Calls
1 Press
press an idle
2
Dial
3
Answering by Ending an Existing Call
Hang up.
1
When the voice terminal rings, lift the handset.
2
Single-Line
Answering by Holding the Existing Call
1 Press the switchhook.
2 Listen for confirmation tone and dial tone.
3 Dial
Answer the waiting call.
4
To return to the held call, press the switchhook twice.
5
NOTE:
to return to the held call.
Answering by Ending the Existing Call
1 Hang up.
2 When the voice terminal rings, lift the handset.
2-16 Voice Terminal Features
.
[HOLD]
or
[SYS ACC-O] [BRIDGED ACCESS]
If you hear dial tone, press the switchhook once more
button.