AT&T System 25 - Manual (Page 41)

Hold
You can hold calls while you attend to other matters and then
pick them up again. After you put a call on hold, you may hang
up the handset without losing the call. To pick up a call you
have placed on hold you must use the same telephone (or one
that shares the same line).
Using Hold
1 Press
The light next to the button of the held call blinks.
2 You can handle other matters without disconnecting the line.
3 To return to the
blinking light.
4 If necessary, lift
held call, press the button next to the
the handset.
5 Continue your conversation.
Hold - Placing Another Call
1 Press
2 Press an idle line button and listen for dial tone.
3 Place and complete the second call.
4 To return to the held call, press the button next to the
blinking light.