sorry maybe my description is not clear enough..
During a basic call, the MSC sometimes need to send a PROGRESS message to the Mobile Station, so as to let the MS stop the call related timers. For example, in state N3 (Mobile originating call proceeding) the MSC will send a PROGRESS.
During my test, in other states other than N3, if the MSC send a PROGRESS message, some certain Mobile Station will send back a DTAP STATUS indicating a protocol violation.
However according to section 5.5.6 in 3GPP 24.008, “At any time during the establishment or release of a call and during an active call the network may send a PROGRESS message to the mobile station.”
Per my understanding, there should have some reason that these mobile stations don’t accept PROGRESS, can anybody explain the reason??