I am currently with 1and1 on one of their lowest end dedicated server plans and while their network is ok, it has issues now and again. What I am looking for is a Dedicated Server host who will install trixbox for me on the box, and who isn't running a network oversold to hell and back as I care far more about latency and jitter than overall bandwidth. I don't need the box managed or anything as I can take care of it, but finding someone willing to shove a CDR into the damn thing and hit enter 3 times is amazingly hard.
Another consideration since you're looking to do VoIP traffic:
Which bandwidth providers are being used to deliver your bandwidth?
It's been our experience that you'll want Either/or/both Level3 and GLBX included in your bandwidth blend as those have proven (in our experience) to be the best providers for that type of service.
Many hosts have a "premuim" blended bandwidth feed and a "standard" feed. Before you buy, be sure which providers are being used for you.
Do you have a preference for where the server is located? (East Coast/West Coast, etc)? I am sure if you contact some VOIP dedicated server providers (or VOIP VPS providers)they can work with your budget.