Calls not disconnecting on PBX side (with voice mail use).

Try setting AT>HFX=1. This will open tip and ring for 4 seconds FXS port once the cellular side hangs up.
If the PBX does not like 4 seconds, then:
Try setting AT>HFX=2. This will open tip and ring for 1.2 seconds FXS port once the cellular side hangs up.