Hello Alpesh,
You won't be able to use the Linphone iPhone app for devices running iOS < 6. After this AudioSession compilation error, you'll encounter runtime errors regarding the xibs.
That's absolutely normal: we've started using the iOS6-specifics (AudioSession objects, strings file-based translations) a few months ago, because it immensely simplifies the maintainability.
The liblinphone SDK, though, is still compatible upward of iOS 4, so if you have to stick to the iOS 5 compatibility, you'll have to implement your own UI.
Regards,