Maybe it is time to replace the phone? Almost any new phone today (other than disposables) are compatible with the global networks. If cost is an issue, I suggest you buy a used phone. It is very easy to buy a great phone that is only 18 months old.

Also, you didn't mention what carrier you use. I use AT&T. I get great service in the USVI for free. I also can get free coverage on the edge of the BVIs: Nanny Cay, Norman, Soper's Hole and Jost because the signal travels well across water from St. John.