user56700 is absolutely wrong.
Congestion avoidance is a mechanism in QoS that allows you to do just that, avoid the congestion before it happens. Microbursts of just a few seconds will cause drops on a 'blocking' switch, where all the ports cannot be forwarded at line rate. Additionally, applying a shaper outbound on internet circuits will allow you to specify max bandwidths for certain classes (voip and the LLQ) to avoid overrunning other classes of traffic. QoS on your switch and router will be different and you will often want both, in conjunction with each other to provide the desired result.
Keep in mind, there is no right answer for how your QoS should look, this is very environment specific.
To avoid explaining QoS here (very very extensive topic) I will just say that auto QoS may be your best option.
There are tons of options for making your goal happen, most phones will automatically mark the traffic appropriately (EF for the rtsp stream and CS3 for control) as long as you trust the markings and disable ip dscp rewrite on the switch, and all routers along the path that you intend to provide a differentiated quality of service you will see the appropriate markings.
Alternatively, you can mark cos on a per port basis. Take a look at this document and remember that after you mark you need to take action on that marking or it becomes useless.