VOIP means Voice Over Internet Protocol which is used to transmit  Voice calls over Internet. There are many applications which can serve the purpose but the only requirement is that both the phones should have same application installed and Internet connectivity (preferably Wifi or 3G). Here are the few applications that can be used on your Android or iOS device to make VOIP calls absolutely free of cost.

Skype:-

Skype is leading VOIP software application  for mobiles which can be used for Instant Messaging, Free voice calls to phones (Skype Installed), Paid (cheap) calling to landlines and mobiles. It is one of the most popular service and recently Facebook has also launched Video calling Service which is powered by skype.

Download Skype Client For Your Phone

Viber:-

Viber is another popular service for making free VOIP calls. When you install viber it will automatically check your mobile phonebook to check if somebody is already using app. It works with Android 2.0 or higher and iOS 3.1 or higher for iPhones. You can also use viber for instant text messaging.

Download Viber for Iphone And Android

Yahoo:-

Yahoo messenger app for Android allows VOIP service. You can also chat, video call and BUZZ using the new app. It is designed to works on all Android phones with Android 2.0 or higher.

Download Yahoo Messenger App for Android

There are many other applications that can be used to make calls on internet. If you have been using any other app then do share your experience.