If they did then every consumer router would make every packet highest priority by default overnight. Prioritization only makes the slightest amount of sense if you have total control.
That way the customer router could tag the VoIP packets as high priority without us having to deeper inspect the traffic. Since it was required by law that a customer could supply their own router this was also publicly documented. However, abuse wouldn't really be feasible because the bandwidth allotted per customer to high priority traffic was rather small.