I agree that the simplest idea is just to use your AT&T phone and add the Passport($30/month) or Passport Plus($60/month) plan while you're gone.

When you are able to pickup a signal from the USVI your phone will show AT&T on it and your calls are just like calling from home. If you pick up a BVI signal, the phone will most likely show Lime or Digicel and the calls will be $1/minute with the Passport Plan or $0.50 with the Passport Plus plan.