Unless you have a lot of phone usage all at the same time, you shouldn't really need to worry about QoS. We have 10 phones on a 5/5mb connection and we've only run into network problems once, and even then it was the ISP's problem.
That being said, you QoS isn't a bad idea to have in place anyways. You'll want to do it on the router side like RC-Installer said. Your network should be set up as follows:
Comcast --> Modem --> Router --> Switch --> Deskphone
We have a Barracuda NG Firewall at our office. Works really well and they were very helpful in getting it set up.
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