implementing the SIP functionality in IOS5

Question

I want to build an iPhone app which has the option to make calls over SIP (VoIP) but at this moment I have no idea how to start.

Does anyone have some information about

Answer 1

I know it's a bit late, but for future reference; An alternative SIP-stack which we had good experience with is the Linphone project.

As with some of the stacks that was mentioned earlier, this stack is very well suited if you plan to code for other OS (e.g Android) as well.