This is happening because the PBX is not receiving a disconnect tone from the LINKSYS. I am sure that the telco provider is sending the BYE. Linksys is not receiving the disconnect tone or not recognizing it.
In fact the extension/gateway is not actually stuck - IVR logs show in this case that the IVR prompt is still playing. It replays itself.
1. Hangup during DR from PSTN (3102's) may cause the line to stick, system required reboot, happens about once every 2-4 days - Sys reeboot is not necessary - If it is what we think, all you need is restart the IVR service. Of course this is not supposed to happen but better than a reboot. This could be a problem with the disconnect tone. Check the disconnect tone for your country. Put this into the LINKSYS gateway in the section PSTN Disconnect Detection. But I confirmed that the Linksys is not sending a BYE to the PBX. This is why the call is still active.
Is CPC (calling party control) enabled on the linksys? This is important especially when voicemail and DR are concerned. Used to detect when calling party disconnects. Set to Yes.
2) DR responses are too long people hang up - You mean that when an incoming call to PSTN, it remains in ringing state for a long time before DR kicks in? In this case do the following - FXO Timer Values (sec) // PSTN Answer Delay: Set to 1 or 0. You will notice a small decrease in time for the SPA to answer.
3/5. Hung DR PSTN lines causes further instability, eventually hangs VOIP and sometimes extensions - Depends what gateway you are refering to. But yes May cause a problem if the DR rings and nothing is pressed. If you press a menu key then everything will proceed as normal. But This is not related to DR only. Try and make a call and forward it to a sip phone directly. Establish call and Disconnect from the PSTN side first. You will notice that the Linksys and the PHONE are still seen as connected although you can hear the disconnect tone. The linksys does not send a BYE to the PBX. The call is disconnected because the CALLED party disconnects and sends a bye. In turn the PBX sends a BYE to the LINKSYS.
4. Transfer of DR at timeout to a call group sometimes fail, logs say call is disconnected - we will need logs for this problem.
6. No yellow "calling" traffic lights on called extensions when calling - we cannot replicate this. Extensions turn to Yellow when ringing.