MKT,
If the SDCCH channels are all occupied, it is indeed possible that the BSC sends a message to the MS, on the AGCH : the “immediate assignment reject”. This is optional feature that is not mandatory !
It tells the MS to re-send a Channel Request after a certain delay (this delay is configureable)
I don’t know if there is a message from BSC to MSC, but if there is, it is NOT linked to the immediate assignment reject.