AT&T MERLIN LEGEND System Reference Manual page 108

Features
System Programming
Related Features
When the extension is available, the call is transferred to the extension
automatically. If the extension is not available before the Transfer Return interval
expires, the call is removed from the Callback queue and returned to the
originator.
If a single-line telephone user with a call queued for a busy station completes a
transfer to a busy pool, the caller hears a fast busy tone. The transfer is
considered complete, and the call is not returned to the originator.
Specify the number of rings (1-6) to the Callback originator before the
system cancel Callback request. The factory setting is 3 rings.
Options -> Callback
Enable or disable the use of Callback for busy trunk pools for Remote
Access users.
LinesTrunks -> RemoteAccss -> AutoQueuing
Automatic Route Selection
Line Request
Callback 2-59