This attribute indicates the last TRM signal received from the ISC.
The received TRM signal can be one of:
Invalid (xxx): this signal should never be received,
Switching Centre normal (101): this signal is allways transmitted
by the ISC when there is no other signal to be sent,
Maintenance release ack(110): this signal is transmitted by the ISC
if it accepts the maintenance release request,
Dcme clear of traffic: this signal is transmitted by the ISC if it has
accepted the maintenance release request signal and now informs
the DCME when all circuits are idle,