There are many people on this board that use Vonage with Dish. I believe they entered *70 or *99 in the dialing prefix are on their receivers. I have Voice Pulse and have no problem using *99.
Adelphia cable modem with vonage, no problems, no prefixes, but I do have my "bandwidth saver" set for highest quality. Don't know if that's required, but if you do have any problems (you probably wont) that's the quickest/easiest thing to tweak.
I'm using a Sipura 2002 on Voicepulse with *99 prefix on my 411 and 622, and works great. Can use the channel 100 interactive services to make changes to my account etc.