AT&T MERLIN LEGEND System Reference Manual page 424

Features
If a single-line telephone with a timed disconnect is used (for example AT&T
models 2500YMGK-PEC3178-NHL and 2500MMGK—PEC3101-ETR),
pressing the switchhook automatically disconnects the call. With this type of
telephone, the Recall button must be used instead of the swltchhook to
transfer a call.
A single-line telephone user cannot transfer an outside call to an outside
number. If the single-line user tries to complete this type of transfer, the caller
remains on hold for transfer and the transfer destination is disconnected.
If any user tries to complete a transfer to an outside telephone number, the
destination outside number is disconnected under the following conditions
(the transfer originator does not receive an error tone to indicate that the
transfer was denied):
The outside line used to make the call is a loop-start line with unreliable
disconnect.
Another inside user joined the call and the call is now a conference call
(which cannot be transferred).
When a call is received on a T1 channel that is programmed to emulate a
loopstart line/trunk and then is transferred to an outside telephone number, it
is not disconnected and remains on hold if the caller hangs up before the call
is answered.
The following types of calls ring at the telephone they are transferred to,
regardless of whether a Voice or Ring button is used:
Calls that arrive after waiting in a Callback or Call Waiting queue.
Calls to busy extensions that do not have the Voice Announce to Busy
capability.
Calls to a telephone programmed to prohibit voice announcements.
Calls to a telephone whose speakerphone is in use.
Calls to single-line telephones.
Calls to a calling group.
Calls to a QCC system operator.
If the system is programmed so that outside callers hear Music-on-Hold while
they are being transferred, and if the person transferring a call hangs up
before announcing it or uses Camp-On, the transfer is considered completed
and the outside caller hears ringback instead.
A single-line telephone user cannot make a voice-announced transfer.
When One-Touch Transfer with automatic completion is programmed and a
call is transferred to a busy extension or to an extension using Do Not
Disturb, the transfer is completed automatically even though the call cannot
be connected. The call does not return to the transfer originator until the
Transfer Return Interval expires.
Returning transferred calls ring at the originating telephone with a transfer
ring (a three-ring pattern). Display telephone users also see the call type
(Return) on the display.
Transfer 2-375