@nony - Wi-Fi technology is an adequate fall back in most situations, but a lot depends on the country you’re traveling in. If you’re in a country that is still slow to adopting new standards and their existing phone lines are weak and decaying, it’s unlikely that a Wi-Fi hotspot nearby is going to be advanced enough to provide a passable way of making a call.
I’ve lived in some parts of Asia where it didn’t matter whether I was using their traditional phone line or the Internet I still had problems with the phone calls.
On the other hand, when I’d stop in Beijing, Hong Kong or Singapore on business travels, the call quality was outstanding. Of course in those places I didn’t really need the Internet for the phone calls. Further, Beijing has strict controls on Internet usage but they do provide GSM so it’s not much of an issue.