AT&T System 25 - Manual (Page 60)

Send All Calls
You can use the button to send your calls
immediately to coverage. If you do not have coverage, the Send
All Calls feature simply turns off the ringer at your telephone for
most incoming calls. In either case, the light next to the ringing
button blinks until the call is answered, or until the caller hangs
up.
Your System Administrator determines whether or not your
telephone will ring once when a call comes in, to remind you
that this feature is turned on.
Turning On Send All Calls
1 Do not lift the handset.
2 Press
The light
Turning
next to this button turns on.
Off Send All Calls
1 Do not lift the handset.
2 Press
The light next to this button goes off.
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