How to implement VoIP + SIP in iPhone? [closed]

I want to develop a VoIP application for iPhone .

But I don't know the basics of VoIP concepts and also if there are any sources or library available that can I use in my application .

So if anyone can provide me VoIP learning resources or library or anything that is useful in terms of VoIP and iPhone it will be precious to me .

If it's still relevant: use pjsip . It's a SIP library written in C. You must compile the source for iphone, it is described in the docs here. After that take a look at a very basic sample application here. It isn't very hard to use even if you don't know C, or how to compile libraries. Which was my case.

UPDATE Please be aware that pjsip uses GPL license.

Another option is Twilio Client which has an iOS SDK. With Twilio you don't have to maintain any of the telephony infrastructure.

(I worked at Twilio)