Everyone everywhere I travel (except the US) seems to use WhatsApp. As pointed out, it's free as long as you have a wireless connection! And as the previous poster points out, you first have to enter the contact in your "contacts" file. But it's super easy and user friendly.
For AT&T users: I find that buying a small data package from AT&T's global passport program is a great investment. $30 or $60 for a month and covers just about anywhere on the planet. I use it for car navigation, phone calls, cruising (in all ways), grindr -- the sixty buck package is hard to use up in a month even with heavy use.