I use ATT DSL in So. Florida and had been on 3/384. VOIPo worked OK but would really go crazy when trying to use the phone while doing any kind of downloading.
A while back, for $5 month more, I upped to 6/564 up and haven't looked back since.
I shuffled among 4 different routers with dd-wrt on and off and tried every iteration of QOS, but no help. Now, with QOS off, no matter how congested my lan is, no trouble at all.

Reason for spelling this out is to indicate that everyone has a different situation and for sure, one size doesn't fit all.