Short answer: No.
If your edge devices do not pass QOS data through to the other end of your calls, all you will accomplish is prioritizing the VoIP traffic internally, on you LAN.
If that’s all you want to use the phone system for, then great.
I suspect you will actually make outbound calls, and for them to be given priority you will need a router that enforces QOS on the WAN port.
I have recently solved a QOS issue for a client using a fairly simple Linksys WRT54GL router. It did not support WAN QOS in the stock firmware from Linksys. But using DD-WRT (or Tomato) I was able to enable this support.
Higher end routers, firewalls and UTM (unified threat management) boxes will definitely support this requirement as well.
That doesn’t mean Windows Server 2003 won’t support QOS. It does support the Diffserv implementation. It just means your traffic outbound to your ISP won’t be given the prioirity you expect along the way.
Hope you have a successful implementation.