AT&T MERLIN LEGEND Release 4.0 Feature Reference page 479

Feature Reference
On an outside call, when the system is using host switch services such
as Centrex, the signal may be sent to the host, depending on the type of
telephone and system operating mode.
A multiline telephone user can use Recall to access host features. The
user can be connected to another party or can be listening to outside
dial tone, ringback, or a busy signal.
A single-line telephone user can use Recall to access host services
only if the system is programmed for Behind Switch mode.
Recall is used by pressing a fixed or programmed Recall button (or Flash
button on some single-line telephones) or dialing the Recall feature code. The
Recall timer, which specifies the duration of the switchhook flash, is set through
system programming. The duration required by the host switch is specified by
the local telephone company.
The Recall timer should be reset if multiline telephone users experience either of
the following problems:
When the user presses the Recall button on an outside call, nothing
happens. This indicates that the interval is too short and should be
increased to 650 milliseconds or 1 second.
In a system operating in Behind Switch mode, when the user presses the
Recall button on an outside call, the call is disconnected. This indicates
that the interval is too long and should be decreased to 350 ms.
Release Differences
Release 1.0 or Release 1.1
Recall can be used on an outside call only if the call has been made or received
on a personal line or Pool button. Recall cannot be used on an outside call
made or received on an SA or ICOM button.
Release 2.0 and Later
In addition to calls made or received on a personal line button or a Pool button,
Recall can be used on an outside call made or received on an SA (including
Shared SA) or ICOM button. This includes the following kinds of calls:
Transferred calls, Group Calling, and forwarded calls received on SA or
ICOM buttons
Automatic Route Selection (ARS) calls and calls made using a pool
dial-out code (on SA buttons) or using the Idle Line Access code (on
ICOM buttons)
479
Recall/Timed Flash