three-party conference call:beep sound continues

[jaheshkhan]

recently we have replaced the 2900 and 3900 series with 8200 and 8300 respectively. After that users are facing the following problem.
In a three-party conference call, if one party disconnects, the beep sound continues throughout the call, which prevents the remaining parties from speaking clearly.
IF two call center agent IP Phones and one mobile are participating in a conference call, and mobile participant leaves the call then other two IP Phones hear continuous long beep tone till they disconnect the call.
At the same if the leaving party is IP Phone then there is no beep sound.

What could be the problem? 

[jaheshkhan]

we have followed existing configuration and copied to new router.. old router they never faced this problem. firmware was upgraded to latest stable version during that VG migration.

[jaheshkhan]

IF two call center agent IP Phones and one mobile are participating in a conference call, and mobile participant leaves the call then other two IP Phones hear continuous long beep tone till they disconnect the call.
At the same if the leaving party is IP Phone then there is no beep sound.

[jaheshkhan · Accepted solution]

After contacting TAC here is the solution : 

Based on the above, our strange issue happens due to the progress indication request that we received from the ISDN which postpones sending RELEASE message in order to release the call immediately after the disconnect request, to fix the issue please run the below command on the VGW:

conf t
voice call disc-pi-off
exit