You do not have to call Tmobile (TMO) or change sim cards you just go. When you land, I always get a txt that says welcome to XYZ you have free data and text, calls are 20 cents/minute. It you want to upgrade to 4g you can (I never have).

TMO has great customer service, you always get to a human fast.

ATT phone work on the same system at TMO so just unlock it and get a TMO sim card in the states.

My last trip sailed from Jamaica to DR with stops in Haiti, no issues texting or calling, but internet was VERY slow in Haiti. You also need to consider that cell phone coverage is not optimized for the water, but for the land.

Look at Tmobile international coverage on their website for more information. Two big plus items, no changing sim cards, so I keep my number and no roaming cost. Downside of not having a local sim card is calling locally when roaming or having people call you, AKA calling a BVI number from a US number does cost. I never found the cost to be much, but there is a cost.

At my suggestion the folks at Active Capt tried TMO in bahamas and hated it as the internet was not fast enough, but they are trying to run a website and blog.