voiceapp on SIP trunk doesn't get fast-busy tone on farend disconnect

[RM-LOCAI]

Hello Cisco Experts,

We've got a CUCM system connected to a 3rd party voice application via SIP trunking. It is basically a conversational bot application. 

The problem is the sometimes callers on far-end (PSTN calls) disconnect the call in between the calls and at this time the applicaiton must receive a response/signal from CUCM to disconnect the call. We can hear fast busy tones in the recordings but it seems the voice application doesn't catch this. Is there any SIP parameter on the trunk or profile we must select in order to send the PSTN fast busy tone to be propagated as a SIP signal over the SIP trunk?

As per the app provider, they need the CUCM to send the SIP BYE message when the call ends which they are not receiving. Is there any parameter or configuration in the SIP trunk to send a SIP BYE message while the PSTN caller hangs up the line.

Below is the flow of the call/connectivity

Caller on PSTN <--> Cisco Voice Gateway <--> Cisco CUCM <-- SIP Trunks--> Voice Application

Any suggestions in this regards would be highly appreciated. 

Thank you

[Roger Kallberg]

A fast busy signal isn’t what you want to get when someone on the PSTN ends the call. What you would expect to get from your service provider is a BYE message. What do you see in the signaling message from the service provider when the far end terminates the call?

[RM-LOCAI]

Hello Roger,

Yes, we need a SIP BYE message to be sent to the voice app via SIP trunk when the caller hangs up the line.

On the PSTN side, these are not SIP lines but ISDN-PRI lines (digital).

[Roger Kallberg]

Regardless of the connection type with PSTN you should get a disconnect message when the far end ends the call. What do you see in the signaling on the ISDN side?

[RM-LOCAI]

upon disconnecting the call, we can still see the voiceport is off-hook on the gateway side and below trace.
As per cisco TAC support, there is no disconnect signal coming from the FXO side. 

[Roger Kallberg]

I thought you said that you’re using ISDN, now you’re saying FXO. What type of connection is it that you have to PSTN?