Go to https://settings.webex.com and select Webex Calling.
From the Calling User Portal, go to Call Settings.
Toggle on Business Continuity.
Enter a phone number to ring when your phone loses connection to the network.
Click the Save.
Calling User Portal | Set Up Your Business Continuity Phone Number
With Business Continuity, you can send calls directly to a destination of your choice if your phone is not connected to the network for any reason, such as power outage, failed internet connection, or wiring problem.