There's 2 ways to do this ... use a app like whatsapp to text overseas but the other user needs to have whatsapp , or download a app like textfree or textplus that actually give u a free number to text overseas .. now if your going over seas both will work on wifi as well
Kik and WhatsAap are both good apps, and their data usage is also low. Textplus although a good app uses more bandwidth hence takes time to update.
I am am traveling in south east Asia right now, so bandwidth is at premium. ( by premium i mean most of the time i am on Edge.)
For text i am using using combination of whatsup and kik, and for calls back to USA i am using Viber.
PS: Magicjack+ came in real handy. i am magicjack fan these days lol.....