You can change a phone assigned to a user. If a phone is not working or can't find the network, you can assign another phone to the user.
First, you'll need to unassign the old phone, then assign a new phone, following the instructions above. Be sure to write down the old phone's MAC address so you can use it when you assign the new device.
When you assign the new device, select Normal Use from the Phone Codec options section. Don't select Multi-User Phone (Hoteling Host); this is currently not supported.