The release of the call in GSM network can take up to 2 s. If you experience longer delay, it can be caused by gateway or by PBX settings. Usually the connection stays active because of simulation of the analogue line. If call is disconnected in the analogue line, you can hear a busy tone. This is not typical anymore in ISDN networks but you can simulate this behaviour and generate busy tone up to 30 s.
On the gateway side you can set following:
If call stays up while being disconnected from GSM side. Go to ISDN parameters and set Busy tone on None. (This will turn off busy the tone generation from the gateway.)
If call stays up while being disconnected from PBX side. Go to GSM Basic parameters and set Busy tone on None. (This will turn off the busy tone generation from the gateway). If call is not cut after this settings, then set this parameter to Disabled. (If PBX requests to keep line active in disconnection message then gateway will ignore this request and the call will be ended)