2N BRI Lite/Enterprise v1.1 - Manual (Page 104)

Troubleshooting 8.2
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Call is not disconnected immediately
 Call release in GSM network can take up to 2s. If you are experimenting
longer delay it can be caused by gateway or by PBX settings. Usually the
connection stays active because of simulation of analogue line. If call is
disconnected in analogue line you can hear busy tone. This is not typical
anymore in ISDN networks but you can simulate this behaviour and
generate busy tone up to 30s.
 On gateway side you can set following:
If call stays up when call is disconnected from GSM side. Go to ISDN
parameters and set Busy tone on None. (This is turning off busy tone
generation from gateway.)
If call stays up when call is disconnected from PBX side. Go to GSM Basic
parameters and set Busy tone on None. (This is turning off busy tone
generation from gateway). If call is not cut after this settings set this
parameter on Disabled. (If PBX requests to keep line active in
disconnection message then gateway will ignore this request and the call
will be ended)