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