Ribin,
There are a couple of different items that you will need to consider before approaching QoS in this scenario.
1.) Since you are running SIP, there are a number of other applications (ie IM applications) that also use SIP. The amount of other SIP traffic that is running on your network may be negligible relative to your VoIP solution. If you are want to limit the QoS on the "legitimate" VoIP traffic, you'll need to be able to "match" on the traffic using an ACL.
2.) In some cases, the DSCP (Differentiated Services Code Point) value will be already set by the application passing the SIP traffic. This is often the "easier" way to do QoS - match on the DSCP bits. If you want to know for sure if these are set, do a packet capture (using Wireshark) for the relevant traffic and expand the IP header.
Once you make these considerations - whether you are going to QoS the traffic based on the DSCP or based on an access-list - we'll need to know what type of device(s) you have on your network that will be responsible for QoS'ing the traffic. If you have an ASA (similar for other Cisco Firewall device), consider the link below for Modular Policy Framework:
http://www.cisco.com/en/US/docs/security/asa/asa82/configuration/guide/mpf.html#wp1143894
If you are going to QoS the flow using IOS, consider the link below:
http://www.cisco.com/en/US/docs/ios/12_2/qos/configuration/guide/qcfmcli2.html
If this answers your questions, please be sure to let us know.
Best Regards,
Kevin
