Audio gets choppy/distorted when playing(after receiving from network) using Audio Unit in iOS

I am writing a VoIP dialer for iPhone/iPad. Now i am writing it's Audio Part. I am using Audio Unit API.

I am recording using recorder callback and sending the full data chunk after encoding the data into G729. And feeding the data back to the player callback after receiving the data from the network and decoding back to G729.

The encoding and decoding is OK(I have tested it). The problem is, audio is very choppy and there is a huge lagging. Can anybody know what's the reason it ?