This has happened to me too, dialing Australian and French mobile phones using Vonage from the US.
We discussed it in our neck of the woods at this thread:
Lay My Hat :: View topic - Phone virus help
Vonage technical support seems, frankly, less than adequate to deal with this. At least they refunded the cost of the call.
I know this thread is old but I thought I should just post the link so that other people searching for this will be able to see all the places where this has been discussed before. It's maddening...