When on the website, you are asked to provide the number of the person you want to reach in Puerto Rico. You then enter your own name and number so T-mobile can let that person know who is trying to contact them.
Once, the T-mobile customer has a connection with the network they will be sent a text message with your name and number.
T-mobile states on their website, "this could take a few days, but they’ll get the message."
Anyone who signs up for the service in the states does not need to be a T-mobile customer, but those in Puerto Rico must be using cell phones on the T-Mobile network.