Its possible to implement VOIP using XMPP. For ios many are providind SDK's which you can easily implement in your code. Among that one of the best is QuickBlox, which is very useful and easy to implement. Its providing P2P video chat, text chat , group chat and file transfer facilities.
But now WebRTC is gaining much popularity for VOIP purposes.WebRTC (Web Real-Time Communication) is an API definition being drafted by the World Wide Web Consortium (W3C) to enable browser to browser applications for voice calling, video chat and P2P file sharing without plugins.
Open Tok is a rather useful product which can be used for VOIP calling in ios devices. They are also providing a basic IOS sdk for easy implementation.
