I can't speak for Skyroam, but we have an unlocked phone and last year got sim cards from mrsimcard.com, one to cover USVI (where AT&T is king) and the other for the BVI. The will deliver and activate the cards to your requirements. Topping off with additional time is easily done.

The only issue was finding a tool to open the sim card slot in the phone. We discovered that a pierced earning stud worked perfectly.