ISDN Disconnect timer

[asterisk18]

I have a CS1000E Rel. 7.5 that I upgraded two months ago (from Rel. 4.5) and I have a problem with the Euro ISDN disconnection timer. Before the upgrade when a caller hung up the phone the PBX user would hear a disconnected tone for about 3-4 seconds and then the switch would hung up. Now this is not working anymore. When the (outside) caller hangs up, the disconnected tone will go on for minutes and the switch will not hung up. It's OK for the simple user, just hangs up manually and it's the end of it. However, there is also a Call Center (SECC 4.2) environment with an IVR. When the customer can not get the info from the IVR then the call is transferred to various services (CDNs) of the Symposium. If the customer receives the info needed and hangs up (within the IVR) then after 15 seconds the IVR transfers the call to the Symposium and when the agent answers, hears the disconnected tone and the statistics indicate abandon call. This is causing a drop on the service level...

Any idea where I can modify this disconnected tone timer? I never have needed to do this on the past. Also, would it make sense to talk to the provider of the PRI lines?

Thanks.