Start by navigating to the Phones menu and selecting the Edit icon next to the phone you wish to unassign.
A pop-up window will appear:
1. Delete each entry in all of the line appearance fields
2. Click Save
This process will keep the MAC in the system for future use, but remove the user from the device:
The final step in this process is to Factory Default the phone. Defaulting the phone will remove any custom configurations added to the device from the previous user, making it ready for reassignment later.