Factory reset a Yealink phone

Factory reset using the phone UI

Press and hold the OK button, this is the button in the centre of the arrow keys which is slightly indented. 

A message appears "Reset to factory setting?" press the OK button. 

Factory reset using the web UI

Find the IP address of the phone. To do this using the handset, press Menu and then Status. 

Go to your phone's IP address in the web browser of your choice (HTTP authentication details are available on the Hardware page).

Click the Settings tab, then click Upgrade on the left-hand side. Finally, click the Reset to Factory Settings button.