I have case with one of our customers, that he has only analog line card TLU44, when I connect it to FXS port I can receive the call but Cant send it, the Cisco phone giving ring tone and the FXS show that it is ringing but the Ericsson does not have any response.
when Ericsson calls cisco the ports shows busy, the other direction the Ericsson port shows no response.
There is a debug command, "debug vpm signal" which roughly translates to "debug voice port module signaling"
Run this debug while a call is attempted and see what the output shows. Either log it to buffer and review buffer: "logging buffered 500000 debug" or use terminal monitor: "logging monitor debug" & "term mon"
You need to ensure that one end if FXO while the other is FXS. FXS is the one that generated the signal/current and FXO receives it and acts on it. FXO to FXO is a dumb back to back connection. Take the following for a test call -
debug voip vtsp all debug vpm signal debug voice cc inout