Not really helpful for the OP, but along the lines of others posting experiences...

I just returned yesterday from 2 weeks at sea in the Indian Ocean. Our cell phones worked pretty much everywhere we were -- although often the satelite service resulted in quite long delays, it reminded me of international phone calls from 15 years ago! My husband and I live in Europe so both have GSM/3G phones (provided by our employer, so we will never see the bills!), and the provider listed usually just said USA -- presumably the cruise line had a connection through the US (US-based line) that we picked up. Wasn't a problem to use, but presumably expensive...

Someone asked about Skype...it would probably work, but we found our wireless internet connection in our cabin (and the wired connection on the ship's PCs in the Library) were VERY slow...not sure the impact that would have on VOIP.