Not able to answer inbound call on FXO

Thanks for all the suggestions which I investigated for potential solutions.
Finally I decided to test the cables which went through several connections.
Using a POT I traced back for dial tone and found that I needed to replace one of the cables in the link.
I had assumed the cables were okay---inbound calls were ringing at the target extension!!!
For the record here are more specifics of the fault symptoms:
Using a mobile place the call to the line terminating on the FXO port.
Dial tone on mobile
2-3 seconds elapses
Target extension rings
Off hook
Audible Ringing ceases on target extension, ringing persists on mobile.
3-4 seconds elapses
Audible ringing on target extension restarts even though still off-hook.

I haven't done the investigation yet but I speculate that perhaps the problem cable was a crossover cable??