Once again, applications like this are hugely confusing.

Whatsapp is one of 100s of Instant Messaging Clients, Viber, mentioned here, is another.

All of these apps are broadly classified as Voice over Internet Protocol (VOIP) services although these particular apps are for short messages carried over the Internet (WiFi). To utilize the apps, you have to have an internet connection.

SMS (Short messaging Services) is a standardized format for phones using 3g/4g/LTE transmission protocols. To use SMS texting, you have to have a connection with a phone service that provides 3g/4g/LTE data service. Apps like whatsapp and Viber don't cross over to 3g/4g/LTE data services.

Buying a plan like ATT Passport (or any international calling plan for any voice/data provider - Sprint, Verizon, etc.) allows you access to international phone and data networks at a reduced charge. If you don't have an international calling/data plan and you connect to an international phone/data service, you are "roaming" and subject to all the excess fees your carrier charges.

Thus, voice/data services and WiFi services are distinctly different. For the purposes of this discussion, if you have a WiFi connection, you can communicate via apps like what'sapp with others who also have a wifi connection AND have the same app as you do. If you want to use SMS texting on your smart phone, you must have access to a local data network (Service provider). You can get this in a number of ways. (1) Buy a plan from your phone service provider that reduces the cost of doing that by "roaming," (2)Buy a Sim card for your "unlocked" phone and a data and calling plan from Digicel. There are other options but to keep it simple here are two.

Here's what I do with ATT: In the BVI you can buy an unlimited calling plan for just this region. It is $30/month. You can call or SMS text between phones in the US and phones in the BVI. I have a Digicel Mobile Hot Spot and a 3gigabyte Data Plan. This gives all my devices (phone and tablets) access to the internet via Digicel's 3G network. I have Viber on my Windows 8 phone. I find I rarely use it.