Whenever an unexpected message, except RELEASE or RELEASE COMPLETE, or unrecognized message is received in any state other than the Null state, a STATUS message shall be returned with cause No.98, message not compatible with call state or message type non-existent or not implemented and the corresponding diagnostic. If a network or user can distinguish between unimplemented (or non-existent) message types and implemented message types which are incompatible with the call state, then a STATUS message may be sent with one of the following causes:
a) No. 97 - message type non-existent or not implemented; or
b) No. 101 - message not compatible with call state.
Alternatively, a STATUS ENQUIRY message maybe sent requesting the call state of the entity (see 5.8.10). No change in state shall be made in either case at this time. This alternative is not applicable to messages using the global call reference.
However, two exceptions to this procedure exist. The first exception is when the network or the user receives an unexpected RELEASE message (e.g. if the DISCONNECT message was corrupted by undetected transmission errors). In this case, no STATUS or STATUS ENQUIRY message is sent. Whenever the network receives an unexpected RELEASE message, the network shall disconnect and release the B-channel; clear the network connection and the call to the remote user with the cause in the RELEASE message sent by the user or, if not included, cause No. 31, normal, unspecified; return a RELEASE COMPLETE message to the user; release the call reference, stop all timers and enter the Null state. Whenever the user receives an unexpected RELEASE message, the user shall disconnect and release the B-channel; return a RELEASE COMPLETE message to the network; release the call reference; stop all timers, and enter the Null state.
The second exception is when the network or the user receives an unexpected RELEASE COMPLETE message. Whenever the network receives an unexpected RELEASE COMPLETE message, the network shall disconnect and release the B-channel; clear the network connection and the call to the remote user with the cause indicated by the user or, if not included, cause No. 111, protocol error, unspecified; release the call reference; stop all timers and enter the Null state. Whenever the user receives an unexpected RELEASE COMPLETE message, the user shall disconnect and release the B-channel; release the call reference; stop all timers, and enter the Null state.