How to start developing an iPhone VOIP application, n which APIs of supported libraries are available?
The iphone has several VOIP applications already available on the AppStore. Therefore, for development, one has a very good chance of approval. Truphone, fring, iCAll are all examples. Also here is the open source SIP stack http://www.pjsip.org/pjsip/docs/html/index.htm
Are you sure that Apple will allow you to create an application that competes with its main source of income from its operator? You can test your concept before chasing it too far.
, VOIP. , .
http://developer.apple.com/library/ios/#DOCUMENTATION/iPhone/Conceptual/iPhoneOSProgrammingGuide/AdvancedAppTricks/AdvancedAppTricks.html
I think Apple does not approve of VOIP applications, refer to Google Voice. Google Voice has been rejected in the app store. There is so much noise in this. You can reference this in techcrunch.