If you are experiencing issues with your Yealink T21PN you can attempt to factory reset the unit.
Please note that this will remove all settings from your device.
On your handset, select Menu whilst your phone is idle.
The menu will load. Scroll down to Settings using the down arrow and use OK to select.
And then use the same method to select Advanced Settings.
You will be asked for a password. If you purchased the phone from Gradwell, this will be the last six digits of the MAC address, which is shown on the label on the rear of the phone.
If you did not purchase the phone from Gradwell, the default password is admin (n.b. please read this article on the importance of changing the web interface username and password on your device).
Now use the down arrow to scroll to Reset to factory and press OK.
You will be asked to confirm whether you wish to reset the phone. Click OK to confirm.
The phone will reset and restart.
This will take several minutes - please do not unplug the phone at any point during this process.