I am assuming you are going to Japan, specifically Atsugi. I might have my reading comprehension from separate threads all jacked up... but...
We used Vonage while we were in Japan, with mixed success. The calling rates to the US were inexpensive from the base TELCOM, and a majority of our calls home were via land line. Vonage was useful for friends and family calling us though.
When I left the Kanto plain a year and a half ago, Americable was the main internet provider on base. Yokosuka was just getting fiber lines for the local higher speed DSL (45-100MB).
Here is Americable's link:
http://site.americablejapan.com/
I am bringing that up, since vonage, skype, or whatever VOIP tech you are using, is going to depend on your ISP and the connection speed you have. High speed internet is not a problem off base in Japan (though there are some funny hoops you have to jump through with the landlord if he hasn't purchased a phone line). On base, the internet speeds have gotten a lot faster. Unfortunately, Americable started offering their own VOIP tech while we were there, and I saw a dramatic drop off in our Vonage performance.
You didn't ask, but also, for cell phones, we used AU and were quite pleased. I had four phones, and my monthly bill for the family was $70, compared with the $200 I am paying Verizon NOW in the states.
Helowifey probably has the best and most up to date gouge on Atsugi I would imagine.