ISDN call not released in IVR


I’ve got an ISDN trunk ( Patton SN4638 ) connected on my FreePBX installation.
In inbound Routes ,if I set an extension or a ring group as destination, everything work good. If the caller ends the call before the extension take it, the extension stop ringing immediately.
But if I set an IVR as destination, if the caller ends the call, the trunk isn’t released until “a” timeout of 30sec. So, the IVR timeout ‘t’, which I’ve set to 5 sec, send the call to the predefined extension, who get a congestion-tone.

Can you say me where I did a mistake or where can the problem come from ?

Thank you,