When some extension is already busy in a call, second call should be routed to the destination that is set up in the call forwarding menu, therefore it will not keep caller waiting and improve customer service.
Model: S-Series PBX, P-Series PBX
Firmware: Not Required
For s-series pbx, enable 'All Busy Mode for SIP Forking' in PBX - General - SIP - Advanced.
For p-series pbx, enable 'All Busy Mode for Endpoints ' in Extension and Trunk - Extension - Features - Call.
By the way if it's the agent in queue that doesn't want to receive second call notification when he is already in a call, please disable 'Ring In Use' option in queue setting for both s-series and p-series pbx.