We have Vonage and an Apple AirPort Extreme. This isn't the happiest possible combination, since the AEBS lacks any sort of QoS capability. As a workaround, I used to have the FreeBSD machine hooked up as a bridge with dummynet giving priority to the packets from the Vonage box. But this is a pretty heavyweight solution and makes that machine part of the critical path for Internet connectivity in the house.
There just had to be a better way.
Turns out, there is.
The Hawking HBB1 is billed as a "broadband booster." There is almost no documentation available on the Internet about how it does what it does. My guess is that it simply gives priority over UDP packets over TCP, but that's just a guess. In any event, one thing I've discovered so far is that it helps to actually tell the box what your Internet connection's actual uplink bandwidth is. The idea is that for best results, you need to take the burden of dropping excess packets away from your modem (because the modem won't care which one it drops), and give that job to the HBB1. The HBB1 has the ability to test the available upload bandwidth, but I've had better results just telling it that our connection has 640 kbps (which is a tad less than the rated value of 768 kbps).
The HBB1 is just a Ubicom StreamEngine in a box. The StreamEngine has been built into many so-called "gaming" home routers. The HBB1 is a way to add this same thing to a router that doesn't have it - like the AEBS.
So you may be asking, why not just buy one of those routers instead of the AEBS. Well, the AEBS supports IPv6, which so far as I can tell no other home router available does.
Anyway, with this device in place, the Vonage box works just as well as it did when I had the FreeBSD QoS solution in place. They can be had for a very cheap price on eBay, so all in all, I'd say it's a worthwhile purchase.