AT&T Door Phone Controller Installation And Operation Manual page 47

Telephone Line In Use With Door When Incoming Call
Arrives
If there is a call in progress between the door speaker and a
person inside the building and a C.O. call comes in, the person
inside the building (not the door speaker) will hear a call
waiting tone. The person inside the building can hook-flash the
phone (or dial 9) and communicate with the incoming caller.
When the call is complete, the person inside the building hangs
up the phone or to speak with the person at the door speaker
again, the person inside the building must hook-flash the phone
(within 4 seconds after picking up the receiver).
Installation — Home/Residential 3-9