Malek,
Checkout the cisco website. Their preferred method at the moment is LLQ/CBWFQ. You can get documents on the cisco website about QOS for voice packets.
The queuing makes use of Diffserv to prioritize the RTP packets. See RFC 2474/5 for more details.The required IP precedence can be set in your voIP dial peer using the ‘set ip precedence’ IOS command.
Hope this gives you some pointers.