[probably_wrong]

> Well, if that's the trick, they don't need your phone nr at all, they can do ip and os check anyways.

Although that wouldn't be 2FA, it's worth noting that Facebook, Hotmail and Flickr will ask for some extra verification if you connect from a different country that usual. So that's probably not a bad idea.

[ZoFreX]

Google do as well - they block logins that they determine to be suspicious, and a separate country seems to be a big factor in that decision.