We have an Audiocodes Mediant 1000B Gateway integrated with Lync Server 2013, and it has a card with 4 FXO ports
When we receive an incoming call from the PSTN, we can answer without problem. The issue comes when the call is finished. If the call is terminated from Lync side, the call FXO port is released. But, if the PSTN side is who end the call, the call does not release and we can see the FXO port busy.
We are in Mexico, so we change Call Progress Tone File with other one made for Mexico (Default file is for USA ). Once we did it, we tried again and the problem was solved. No matter who end the call, the FXO port was released.
Here’s the link to Download Mexico Call Progress Tone File
I hope this help you