AT&T MERLIN LEGEND Release 3.0 Programming Summary Manual page 169

Fax
Fax
Use this procedure to add a fax machine by assigning the extension jack
used to connect the fax machine. To remove a fax machine and free the
extension jack for another use, you must remove the extension jack
assignment.
In addition, you can specify the extensions to receive a message-waiting
indication (MWI) when a fax transmission is received, and specify the length
of time before the system registers that a fax has arrived and sends the
message-waiting indication.
NOTE:
Do not use this procedure for fax machines connected to analog multiline
telephones with a General Purpose Adapter (GPA). In a GPA configuration
features cannot be assigned to the fax independently of the telephone.
A maximum of 16 fax machines can have the Fax Message Waiting feature.
Additional fax machines (more than 16) can be installed, but these machines
cannot have this feature.
You can specify up to four telephones to receive the message-waiting
indication when a fax transmission is received. Note that fax machines can
only send and not receive message-waiting indications.
Auxiliary Equipment 11-5