Issue description: The call is disconnected from VoIP side and it takes up to 30 seconds before the call is disconnected in GSM side. This typically happens if a voice mail is reached. If you make a test call to your cell phone, you can hear a busy tone.

This behaviour can be caused by PBX settings or by 2N VoiceBlue Next settings.

Please check following page:

Generate busy tone to GSM/UMTS is used to simulate an analogue line where you get a busy tone after disconnection. If you want the immediate disconnection, set this parameter to Disabled.

Similar settings could be necessary on PBX side.