Calls between ipad users

What's the best App or software to make calls between ipad users???

And I'll like to know what is MobileMe exactly? Does it work on ipad???

My wife and I use Whistle.

I also have whistle and the little ad at the beginning is very so short....works great!

Is there any way to change my user name on this site??

The correct spelling is MOZZI.....not Mossi . Somehow, I didn't notice when the spellchecker made the decision.