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.
When I connect my Nokia 22 to an extension line and configure incoming calls to mode B, and program an extension number for the incoming call to be directed to, the incoming call rings the extension, but when I pickup the handset and listen, I just hear DTMF tones, and the Nokia 22 continues ringing as if the call is not completed. I am using a NEC PBX. What is wrong?