Just found an iphone app Viber-Free phone calls & text.
An excellent app to make free phone calls and send text messages. Basically works over wifi or 3G like skype, so you do need to have a basic plan with service providers or use it over wifi. Works on 3G and above. Have no idea on iphone 2 as dont have one...
Have tried it and found it to be good.
Cheers

But doesn't the other person must have viber installed too?
I use whatsapp and thats the app that most users have, at least on my phonebook i have 5 contacts using viber and lets say 45 using whatsapp.

Yes the other person also needs viber installed, however like whatsapp viber uses your telephone contact list and will show who all are using viber. Once you have viber send invite to your friends from viber.
Moreover whatsapp is no longer available on itunes. If you have it fair enough but new users cant download it from itunes anymore...
Viber lets you make phone call and i found call quality to be good.
Cheers

just like any other messaging/calling app, in order to obtain free services, both parties need to have the app installed, be it Skype, Viber, Whatsapp, Google Talk etc.
Viber and Whasapp use the user phone number as identification, hence making their usage one step lesser by removing the requirement of exchanging usernames, BlackBerry PINs or its likes...
Whatsapp is truly multiplatform being available for Android, iOS, BlackBerry and even Nokia... however it doesn't provide free in-app calling functionality...
Viber is available for Android and iOS and provides free in-app calling functionality... a BlackBerry version is under progress...
both provide free in-app text messaging and multimedia exchange...

Viber has been available for iOS since a few years now... and now for Android since a few months...

Been trying to use Viber for years, I find it less than useless unless you like the sound of your conversation to mimic what fish must sound like at the bottom of the aquarium.

Unfortunately Viber isn't available on all platforms yet! I hope it becomes available soon...

Saw this on viber website
The first enhancement you'll notice is improved voice quality thanks to the new voice engine in Viber 2.1. This update will provide Viber users with HD quality VoIP calling and will improve call quality over networks with low bandwidth such as EDGE networks.

Suggest you to update your app coz as i said before found voice quality to be good and not barely audible.

There is an addon as well for Viber on android (I don't know for other os).
It allows you to call non-viber number/contact for cheap.

i.e. call to landline Indonesia (jakarta) is only 0.017 chf/min.

this app is pretty old. and i find it ok when having 3g and/or wifi.
quality is ok, as long as the internet connection is good enough.

In my experience Viber messaging is good but voice only seems to work well iPhone-iPhone, not iPhone-Android

I use viber between iphone (myself) and android (OH). It works as a dream and quality is impressive. This between office and home using ADSL.

Provided my other half is also in a wifi zone it works well. If he's walking around the mean streets of London then it can sometimes drop or echo. I'm always on wifi when I use it, as I have no data plan.

To the guy who said whatsapp isn't available - that was temporary, I just downloaded an update.

As for viber, it's ok, but I can't see why you'd use it these days if both parties are on iOS (assuming wifi, of course). Just do a FaceTime call - keep it in your pocket if you don't want video, and the audio quality is higher!

Not everyone runs out to throw away a working phone to replace it with a slightly newer version.