If you ever find yourself locked out of a Yealink phone, and do not know the admin password, you can quickly reset the phone to Factory Default Settings.
Simply press and hold the 'OK' key on the face of the phone for 10 seconds. This will prompt you to accept or decline a factory reset. If you accept, the unit it will restore itself to factory default settings - without the need the admin password.
The entire process takes roughly 30 seconds.