since the correct extension rings, it means that N22 has answered the incoming call, set the hook off (closed the hook-switch) and sent the re-programmed number to the PBX. If the A subscriber still hears the phone ringing it means that it has not got the 'B-answered' signalling from the gsm network.
I think it would be good to consult your gsm service provider to check the above mentioned issue. Simple test ofcourse is to try with SIM card(s) from some other operator. If it is possible, also a test with different PBX brand could give additional information.
M2M developer support team