Viber, an iOS telephony app, might mimic Skype in functionality, but with the addition of free
text messaging in the latest 2.01 version it might even make you
switch to its services. Unlike Skype, Viber integrates tightly with
your iPhone's phone book, so you don't have to keep a third-party app
open at all times. In addition, you also get unobtrusive notifications allowing you to continue what you're doing without being distracted.
After installing it, you can call and text other Viber users for free and push notifications won't let you miss a message or a call. The new version adds a fresh polish to the user interface. So why don't you check it out on the App Store (an Android version is in the works as well), the voice quality is pretty decent and it's free. Nothing can beat that, can it?