I don't know. This is on your CDG card? So I would be looking closely at the LWPAR. Get the LWPAR assigned in TDCSU, and examine the config with LWPAR AMO.
This is where an ISDN tester is useful, to listen on the circuit and find out at which end the voice is lost.
You had a working DPNSS on the other circuit of the card I think. So you should compare the LWPAR with that one, and if you can, swap the circuits over and test again for the problem.