Here's how to Factory Reset your VoIP Phone:

Snom 3xx Series

Since firmware version 8.7.3.7 on the Snom 3xx Series it is possible to reset the phone using the key pad by pressing the following key sequence:

* Volume Up * Volume Down #

To set the phone back to "Admin mode", press the Settings key:

Firmware versions lower than 7.X: Select "Admin Mode".
Firmware versions lower equal or greater 7.X: Enter "Maintenance" and select "Admin Mode".
On the prompt enter your administrator password, if you have set one, or the default password, which is "0000" (four zeros), and confirm.