Is there a setting that is the SIP wont register, ie internet down, it defaults to the PSTN.

Eg i call using SIP 99% of the time. One day the internet is not working, wife picks up phone, and can still call me in the office, but it automatically uses the PSTN???

The PSTN relay options are for 999, 112 etc that Have to go via PSTN, if i put the office number in there, it will always use PSTN. I only want to use if SIP down.

Thanks