AT&T cell service work best on St. John. Sprint second best. Talk, text and data are all free. Verizon cell service is spotty at best. Text and data are free; however, you will get little to no service on island. Verizon calls cost approximately $1.99 a minute. Your smart phone or iPad will work on the Wi-Fi provided in each unit.
Some parts of the island do not have cell service at all. Other areas connect to British Virgin Island cell towers, which can be quite costly. It is a good idea to switch on airplane mode while exploring the North Shore and Coral Bay.
Check with your service provider to confirm that they provide service in the Virgin Islands and that you will not incur any roaming charges before traveling.