AT&T MERLIN LEGEND Release 4.0 Feature Reference page 567

Feature Reference
In Release 3.1 systems and later, an extension can have trunk-to-trunk transfers
blocked through system programming whether or not the trunks involved are
programmed for reliable disconnect.
In Release 3.1 systems and later, all extensions are factory-set to be restricted
from making trunk-to-trunk transfers. Extensions that should not be restricted
must be individually programmed to allow trunk-to-trunk transfer.
!
Security Alert:
Any changes to the trunk-to-trunk transfer setting (especially for
voice messaging ports or remote access ports) must be considered
carefully in order to minimize the potential for toll fraud. See
Appendix A, "Customer Support Information" for security
information.
Disable Transfer on Single-Line Telephones
The system manager can disable the ability to transfer calls on single-line
telephones in Release 4.0 and later releases. This is done by removing all but
one SA or ICOM buttons from the telephones, in centralized telephone
programming. Any feature that relies on the use of a second dial tone also does
not work on any single-line telephone with transfer disabled. This includes the
features Account Code Entry, Call Pickup, Call Waiting, Conference, Privacy,
and Transfer.
Considerations and Constraints
Calls transferred to outside numbers may vary in transmission quality.
The ability to transfer internal calls to outside numbers cannot be specifically
blocked for an individual extension. However, calling restrictions or Disallowed
Lists can be assigned to individual extensions to prevent outward or toll calls.
When an outside call is transferred to an outside number (trunk-to-trunk
transfer), two outside lines are used for as long as the call is in progress.
When a call is transferred to an outside number, the system does not recognize
the transfer until a dialing timeout occurs. Avoid a delay by dialing
dialing the telephone number.
When you try to complete a transfer to an outside number under the following
conditions, the call to the outside destination is disconnected:
The outside line that the incoming call is using is a loop-start line
programmed for unreliable disconnect.
Another inside user joined the call and the call is now a conference call,
which cannot be transferred.

after
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Transfer