If you are experiencing issues with your Yealink T18PN you can attend to factory reset the unit.
Please note that this will remove all settings from your device.
On your handset, dial **90#. Note down the IP address that is read out to you.
Using a web browser on a PC connected to the same network enter the IP Address, you will then be prompted for a username and password, by default both of these values are set to admin (n.b. please see this help guide if you have purchased the phone from Gradwell).
If you did not purchase the phone from Gradwell, please read this article on the importance of changing the web interface username and password on your device.
You will then see the Yealink configuration screen. From the menu click Upgrade and the Basic. Click the Reset button and the phone will reinitialise.