Viber is a free Android and iPhone app which lets users make phone calls and send texts to other Viber users.
Instead of using your voice minutes, Viber makes use of any available wi-fi or 3G network to communicate. There are no fees for using Viber apart from what you spend on data while on 3G, so you're essentially using your network carrier as a dumb pipe.
Using the app for phone calls two or three times a day didn't significantly eat into my data plan, but it would be nice if the app would show me exactly how much data I used for each call.
Viber is pretty similar to other VOIP services like Skype or Fring, but the beauty of this app is its simplistic approach to user accounts.
Instead of creating a user name and password, you login straight away with your phone number. Viber checks your contact list for other Viber users and gives you the option to dial them from the app.
I've found the app is best used to keep in touch with a small group of friends with whom I am in regular contact. This has significantly cut down the number of voice minutes I need in my plan, and almost removed the need for texts.
Viber is a great app, but it has its downsides.
The most obvious drawback is that if the person you're trying to reach doesn't have access to the internet, the app is pointless.
With both the iPhone and Android version, I've had some major issues with bugs. Mostly it's buttons that don't do what they're supposed to, but I've also had calls hang without any warning.
I won't be throwing away my phone plan anytime soon, but this app does show potential and is particularly attractive for people travelling overseas on business.
3.5 Stars. Free