The following is the configuration I use to apply QoS to my VoIP service with Callcentric. Basically, it gives those packets priority bandiwdth in the event my upstream is saturated. This configuration was built on IOS version 12.4(24)T4, but should work on all but the oldest code versions. If in doubt, check the feature navigator at Cisco. Please feel free to use this in your own setups.
First, the ACL...
The following ACL should match the packets from the DTA to the Callcentric SIP servers. It was created by querying the SRV record "_sip._udp.callcentric.com". Even though I used a named ACL, a standard or numbered extended ACL will work just fine.
ip access-list extended Callcentric-VoIP-Hosts
permit ip any host 204.11.192.22
permit ip any host 204.11.192.23
permit ip any host 204.11.192.31
permit ip any host 204.11.192.34
permit ip any host 204.11.192.35
permit ip any host 204.11.192.36
permit ip any host 204.11.192.37
permit ip any host 204.11.192.38
permit ip any host 204.11.192.39
Then the Class Map...
Here's the Class Map that I configured to classify the packets bound to Callcentric; as you see, nothing fancy.
class-map match-all Callcentric-VoIP description Outbound packets to Callcentric match access-group name Callcentric-VoIP-Hosts
Next the Policy Map
I only have one line of VoIP using the G.711 codec, but if you plan on making simultaneous calls, you will want to adjust your policy accordingly. I've given that one line 128kbps since I have plenty of upstream bandwidth, but realistically, you should give each call about 80-85kbps.
policy-map Outbound-QoS-Policy
class Callcentric-VoIP
priority 128
class class-default
fair-queue
!
And finally, it is applied to an interface.
Once you have the policy map created, you must apply it to your WAN interface. In my case, that interface was FastEthernet0.
interface FastEthernet0 service-policy output Outbound-QoS-Policy !