It probably differs depending on each store and what app they choose to do the fansign on so I'd suggest to read the small print and winner requirements before going forward.

Personally, I've successfully attended a Japan fansign using my Japanese friend's address and home phone. I had won it through their official store on Skiyaki. There wasn't any need to confirm my number or anything instead I was given a code after winning which I then had to enter into WithLive (the app they used for the fansign). For the small print, all I had to make sure was that the name on my Skiyaki account matched with my WithLive one. I managed to win another fansign a few days ago and the process is still the same.

I also know a friend who managed to do a Japan fansign using the address and phone number of a forwarding company haha.