Having had various scenarios where an existing configuration of the phone system cannot be overwritten or reconfigured. I found a simple solution to restore to factory settings and could reconfigure the telephone system and assign the right IP Address etc. to the phone.
Here are the very basic steps to have this done and this is applicable to the 3XX series. Press in the following order as shown below.
“*”, “Volume UP”, “*”, “Volume DOWN”, and then “#.”
If executed correctly, it will reboot and reset the telephone system.
Handy link: https://www.3cx.com/sip-phones/factory-reset-snom-300-320-260-370/