Comment Forget QOS (Score 1) 362
QOS is not what you need to focus on.
Latency in this application will kill your sound quality far more than a few dropped packets. Optimally you'll want to be under 300ms for things to be manageable.
The other main thing to look for is a CODEC you can use with your chosen provider that uses as little bandwidth as possible and supports loss concealment. You need to worry about those two factors long before QOS becomes relevant to the equation.