If a user locks the phone via the keypad menu, there is no way to unlock it except to factory reset it. You should try some obvious possible numbers such as the extension itself, that number backwards, and numbers that may be obvious to the user who locked it. Failing that, you must factory reset it via the web interface.
Reset via web UI:
Press the up-arrow button to show the IP address. Log into the phone on any web browser by typing the IP address into the URL bar. The password and username are both "admin" (some phones may not ask for a username). Click on the "Maintenance" option near the top right then "Upgrade and Provisioning." Select "Factory reset" and then "Reset."
Once the phone reboots, follow the "configure settings" part of this document: