We had Vonage in the US and it worked fine. Sometimes you get an echo or delay which can be annoying.
When we moved to the UK our only option for internet was via a BT phone line, so we had to get a UK number that way. However we brought our Vonage box with us and it works just the same as it did in the US. Specifically we have two phones - one for the UK with BT and one for Vonage that gets us calls to the US and 60 countries for a flat rate. Even caller ID works. Calls to our old city - Tucson - are local calls, just like before.
The benefit of this is that all my wife's friends and relatives in the US can make a local call to reach us, instead of an international call.
HTH, Andy