Bora Bora is part of French Polynesia. The only active network in French Polynesia is Vini. Here is their 3G coverage map, including Bora Bora. Roaming is terribly expensive there.
P.S. I have no idea why T-Mobile charges $4.19/min in French Polynesia. Telna only charges 30¢-60¢/min. Vini has a local prepaid SIM kit for the equivalent of $20.