Vonage works globally if you have a wifi signal

No, it only works if you've got a stable signal that has good throughput.
Which proves to be quite a challenge in the assortment of location I've
tried, albeit within the limited confines of Chesapeake north of the
Potomac. I'm sure signal coverage varies greatly in different areas but I'd
never assume it'd (wifi) be reliable enough to depend on it being present
and usable. Cellular data services are often more reliable but sometimes
slower (usually) and not without their billing issues. So using a p2p
shared system like skype could potentially be a real problem.

The nice part is there are choices. With today's level of deployment
there's not enough to make it reliable but at least it's *possible*. Adjust
your expectations accordingly.