Here’s a quick short post on How to make free voip calls using an Iphone or Android.
Iphone and Android phones ssupport loads of applications that allow you to make free phone to phone calls to other smartphone devices. But here is the trick; Both the devices should have internet i.e. wifi/3g/2g  enabled and should be running the same applications on the phone.

Applications that support free phone to phone calls :
Skype:
Skype is world’s most popular PC to PC telephony service with more than 350 million users across the globe. Well thats old news; Good news is Skype has a mobile app. You can make free call to another user who is on skype using your mobile phone. If your friend is also a smartphone user (Android or Iphone) you can make free calls to them.
Download skype for your phone here :
http://www.skype.com/intl/en-us/get-skype/on-your-mobile/download/
Viber :
Viber is a mobile calling service that works on mobile phones with wifi & 3g internet. Viber not only supports free calling but also supports free text messaging to other viber users. You can also make Viber as a default dialer for your android phone.
Download Viber for your phone here :
http://www.viber.com/

Yahoo Messenger :
The yahoo messenger app for android also supports free calls to other devices running yahoo messenger.
In order to enjoy superior  call-quality you should be using 3g, However 2g Edge networks of Vodafone & Airtel are good enough & give good speed to support voice call quality.