How to implement VoIP + SIP in iPhone?

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)