iOS: Open Source VoIP/SIP Objective-C Code [closed]

I have been tasked with investigating the feasibility of writing an iPhone App to access our internal VoIP/SIP systems.

I've never coded anything close to VoIP before. Are there any open source VoIP/SIP libraries or examples in C or Objective-C?

An iOS App that I can skin and add our required features to (mainly UI related) would be the holy grail here.

You may take a look at siphon (http://code.google.com/p/siphon/).

From their homepage:

Home of the World's first free SIP/VoIP application for iPhone and iPod Touch 1 and 2.

Siphon SIP/VoIP project is the first in his category that works on iPhone and iPod Touch 2 with headset for all SIP providers. It is a native application approved running on 2.X using internal micro/speaker and headset.

The Application supports the SIP standard, preserving compatibility with hundreds of SIP providers and offers a GUI which preserves the apple design of native iPhone applications.

PORT SIP

If you are a new comer on VOIP i would suggest using simple sdk's like PORTSIP
It is free download.you can check the app and make calls and also play around with the call.It is payable only if you want to use it for business.PORTSIP sdk is very easy compared to other sdk's or open source projects.

-ves Not open source,offers less flexibility

LINPHONE

After you get hold of this you go for the open source projects Linphonen/PJSIP etc. Linphone offers high quality sound but is very complicated to integrate and very less documentation is available.you will have to build the project first.http://shallwelearn.com/blog/build-linphone-for-iphone-and-ipad/ (for IOS)

-ve s Very poor documentation

-PJSIP

Your best option is PJSIP which is very good with documentation and offers everything.Because you get code from scratch you can do anything with the code. I highly recommend PJSIP.But it is difficult to directly go and devolep in PJSIP ,what i would recommend is do sample stuff on simple projects like portsip and go for PJSIP

Although it's rather old thread, for reference I add here also pjsip: http://www.pjsip.org that has a quite mature iOS port nevertheless it is written in C and its API is also in C.

UPDATE as of 06/2021: please note that this answer was originally written 9 years ago. I completely off from VoIP development now and can't take any responsibility wether pjsip is still working on iOS or swift.