FXS port staying off-hook when remote call hangsup

I have a Yeastar TA800 FXS Gateway that is providing analog trunk CO appearances for our legacy Samsung iDCS100 PBX.  We have an installation of FreePBX version 14 on the same LAN with the gateway that connects to our SIP provider.  The FXS Gateway does "port register" for each of the ports with our FreePBX.

The FXS Gateway is working and allowing our legacy PBX to dial out and receive SIP calls via the FXS Gateway ports that are simulating the PSTN Loop Start CO lines.

The issue is when a SIP call that is external to the FXS Gateway hangsup and our Samsung PBX SVMI voicemail auto attendant is on the line.  The auto attendant does not recognize the call has ended and does not hangup.  The FXS port remains off-hook for 10 minutes.

When an external caller hangsup, the FXS port hears a fast busy signal, but does not actually do any sort of audible disconnect that I can discern. 

Yeastar support said the following, "For our TA device, currently we have two hangup signal to the end device connect to it. One is busy tone signal, and the other is polarity reverse. When the caller hangup, we'll off the end units either of that two signal and expect the end unit can detect it and hang up itself, otherwise the end unit will still keep alive and cause the port off hook. So please help us check what kind of hang up signal your legacy PBX accept and change to that hang up signal."  I have tried changing the TA800 port to "Polarity Reverse=YES" with no change in the hangup behavior.

I am disappointed that the FXS port does not disconnect first on external caller hangup, but rather waits for the device connected to the FXS port to go On-Hook.  I would appreciate some advice from some of the more experienced here to get my FXS port to go On-hook when someone hangs up on the voicemail auto attendant.


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