You can reset your Konftel 800 to factory default. You can do the factory reset on the phone after you log in as the administrator.
(Rebooting to factory settings erases all the settings including the account information)

Settings > Admin login

Log in to the phone as the administrator.

Settings > Phone > Scroll down and Tap Factory Reset

The phone shows the following message: Reset configuration to factory default. Press OK to confirm. 

If you need to perform the factory reset without logging in as the administrator follow this guide below.