DoS is a serious concern in VoIP networks. Its unlikely a vendors voip code will be virus infected (but you never know). As mentioned by Lansing you could separate your voip servers on a different domain and limit access. To achieve good voip quality you will most likely (it's recommended) be setting up separate VLAN's on the lan switches and if possible feed the trunk uplinks to separate ports on your routers. The routers and switches will need to be set to prioritize the voip.
Most people think of voip (with data) only via the links connecting sites of the wan. If you are actually thinking of voip phones then thats more like the switch and router settings mentioned earlier.