you just call any mobile phone number and choose "video call" instead of "voice call" - the person you call needn't be on any internet chat service like skype or thelike, let alone be on the same network you're on or be online.

as long as they have their phone switched on and are on a 3G network (which is true for really anyone i know), it "just works". like phone calls "just work" while a skype voice call might not. (at least you'll never reach me on skype.)