As the phone and cable companies begin offering their own VoIP services in real volume, they plan to "tag" their own VoIP packets so that at least within their own networks, their VoIP service will have COS (Class of Service) assignments with their routers, switches, etc. They also plan on implementing distinct Virtual Local Area Networks (VLANs) for the tagged packets.
Tagged packets get both less restrictive rules for passage and a private highway lane to drive on.
The net effect is that any packet that isn't tagged will only get "best effort" service, which means whatever is left.