If you think WhatsApp is great, this Viber might be better. You can make phone call or SMS to iPhone users that has the Viber app. Once activated, you can use your existing contact list to make phone call or SMS.
The Viber app is currently free. My friend had tried it and the quality is good. Of course, do bear in mind that it uses WiFi or Data for the call.
So, if you are overseas, you can make phone call to Viber users free. From the website, it says that Blackberry and Android version is coming soon.
Watch this short video on how it works.