making a voip application for iphone?

How can we implement the voip functionality over iphone .Is there api or framwork or any third party library that runs voip for my obj开发者_运维技巧ective c platform what will be the data received format for the calls .And can we track the calls i.e history,duration etc.Are these feature avaliable in the apple sdk developer for the iphone .

links or code will be more helpful !!!

Thanks,

Check http://code.google.com/p/siphon/ Based on www.pjsip.org

Another one is linphone, http://www.linphone.org/eng/download/git.html