Plug in the phone and let it power up completely. The following screen will appear which says "out of service press any key". Press any key on the phone, once the key has been pressed this the phone will talk to the user.
The phone will then prompt for a service number, please enter in the extension. The next page will ask for a password please enter "0000".
If the phone message/instruction is "This extension is invalid" or something similar, then please ensure the correct extension was typed in.
The phone may reboot and go through the 5 step process.
It should now be ready for use.