If VOIP doesn't run on your network because you've oversold your capacity,
no amount of QoS is going to put the quality back into your service.
People will find better ISPs. If you deliberately set QoS to favor your
services over a competitor, whom your customers are also paying for
service, you'll be staring down prosecutors, at some point. It's
anti-competitive behavior, as you're taking deliberate actions to degrade
the service of a competitor, simply because you can.