AT&T MERLIN LEGEND System Reference Manual page 437

System Applications
VMI Ports In/out of
Service
Mode Codes
When a calling group call to a VMl port is not answered within 30 seconds, the
call is sent to another available VMI port in the calling group or is queued back
to wait for an available port in the calling group.
For an integrated VMI port, the control unit sends mode codes to inform the VMS
that the port is out of service. The VMI software marks the unavailable port "out
of service," and the calling group software logs it "out of service." If all VMI ports
go out of service, a programming logic inconsistency (PLI) is generated.
Every ten minutes the system tests each out-of-service VMI port. If the port
responds to the alert, the VMI software marks it "in service," and the calling
group software logs it "in service." For integrated VMI ports, the control unit
informs the VMS by port-in-service mode codes.
Mode codes are streams of touch-tone tones that carry information from the
control unit to the VMS connected to the integrated VMI port. The two classes of
mode codes are Call Information and Other.
Call Information mode codes
direct inside access
direct dial access-line/trunk
call coverage-internal station
call coverage-external (line/trunk)
call coverage-other
Other mode codes
leave word calling
refresh message waiting LEDs
port restored to service
port taken out of service
day service
night service
Voice Messaging Systems 3-5