VoIP is not supported on FXO to FXO calls
Valid calls:
FX0 to FXS
FXS to FXO
FXS to FXS
Please have in mind that when both parties hung up, there is no way to tell that the discussion has ended because there is no signal comming out of the telecom equipment connectedto fxo ports.
One way to detect the end of call is to set a time out for a long time with silence detection. This is not supported with cisco IOS.
You may have just one option: change the modules from FXS to FXO in the on side of the link.