AT&T Door Phone Controller Installation And Operation Manual page 76

Telephone Line In Use With Door When a Call Arrives
Placing a Call Through the C.O. From Inside Building
If there is a call in progress between the door speaker and a
station user and a call (C.O.) comes in, the person inside the
building (not the door speaker) will hear a call waiting tone. The
station user can hook-flash the phone (or dial 9) and
communicate with the incoming caller. When the call is
complete, the station user hangs up the phone. To speak with
the door phone again, the station user must either hook-flash
the phone (within 4 seconds of picking up the receiver) if
installed for Trunk Saver Mode or dial "1" if installed for Port
Saver Mode.
To initiate a call through the C.O. in the Trunk Saver Mode, the
person within the building simply takes the telephone off-hook,
accesses the trunk connected to the AT&T Door Phone
controller, and then dials the desired phone number. This is
normal procedure for placing an outside call.
To initiate a call through the C.O. in the Port Saver Mode, the
person within the building simply takes the telephone off-hook,
then accesses the trunk connected to the AT&T Door Phone
Controller. At this point the caller will hear a dial tone from the
Door Phone Controller. Dialing a "9", or waiting 4 seconds, will
switch the caller to the C.O. and the regular phone number is
dialed.
Installation - No Dedicated Trunk/Station Port 6-11