We only have Vonage around these parts, from what I know, and of the two people that use it, I've used the phone at one of their houses, and talked to both....it sounds like they're talking on the OLDER (think early 80's) cordless phones.
The only downside is that should they ever have to call the cops, they may run into problems, and if their 'net goes out...but both things are most likely not going to happen often, if at all around these parts. YMMV