Hello,
From what I understand you have done a factory reset of the phone and now you want to configure it from scratch. Once the phone has received an IP address, just type the address in a browser and then the web gui will appear. From there on the right side of the screen you'll see a link Admin and Advanced. First press the Admin one and next the Advanced one. By doing that all possible options for configuration will be present in your web gui. Then on a specific line (extension) configure the proxy server, username and password given to you by your VoIP provider or local PBX. Once done that just save the settings and you'll have an extension or extensions working as expected.
If you need any more information please don't hesitate to contact me again.
Best Regards,
