Use SIP in iPhone app

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 this topic, or maybe a demo project which I can us开发者_StackOverflowe to implement the SIP functionality?

Thanks in advance!

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You could check out siphon.

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For this project you'll need a SIP server to handle SIP requests from the iPhone clients. Yes, not so simple is it?

There are open source SIP implementations (like this one) which you can easily find googling.

Of course, you can also make your own implementation of the SIP protocol (RFC 3261).