How To: Assign an Extension to a Device (Phone)

Navigate to the Features Tab and click Devices on the left hand side.
Select the Extensions selection next to the MAC address (Phone) you want to configure.
Click to highlight the extension you would like to assign to the phone.
Select the > to move the extension from the Available Extensions list, to the Assigned extensions list.
Select the < to move the extension from the Assigned Extensions, to the Available Extensions list.
If you want to assign multiple extensions to the same phone, continue repeating steps 3 and 4 until you have the desired outcome.
Once you have the desired configuration, click Save.
Go to Devices and click Deploy Changes.
Once you see the message "Your Changes are now being deployed. It may take several minutes for the changes to take affect." hit OK
Reboot your phone.