If you try to press and hold the OK button but are prompted with a username / password for Factory Reset, you can complete the following procedure.
- Press and hold a line key
- Set line key Type to Key Event, Key Type to Zero Touch, Label to password

- Press the line key, press Setup, Next

- Enter the following URL
https://p.sipvoice.com.au/yealink/1.cfg

- Press OK

The phone will now reset config and username / password will be admin / admin.