I know that using my AT&T iPhone overseas leads to expensive international charges. But is that still the case if I use the phone with a VOIP service like VIBER that uses the internet instead of cellular?(VIBER is like Skype, but it uses your regular phone number rather than a special ID. It's free for calls between an AT&T iPhone in the U.S. and an iPhone located overseas with a local calling plan -- e.g. a Danish plan in Denmark -- when both iPhones are using the VIBER app. The question is whether the call would be free if the iPhone located overseas was using its AT&T U.S. number.)