I have used videophone-capable mobile phones (I have to for work) but at the moment the calls are so expensive that it's never been for very long, just to test whether it works really, and I certainly wouldn't use them internationally. You could do videophone through wi-fi too on modern smartphones, but again it's one of those features that I know about but haven't gotten round to trying.

Another thing though is that phones tend to be trickier to set up, SIP settings are a pain. If video calling is going to be mainstream it shouldn't be any more difficult than just entering a username and password, which is all that you need to do on tablet-to-tablet video calls.