When I was in Nassau I bought a phone card for $5. They also had $10 & $20 cards. They sold them in the Festival Bay building which is the entranceway into Nassau from the cruise ship dock. I remember when you entered the building I went all the way to the left a saw this window like a drive up window at a bank. Behind the window was a woman that sold me the cards. Back then (Nov. 2004), it costed 51 cents/minute to call the U.S. using the card. The phone card would work on the payphones in the hallway around the window.
I hope this helps. Have a good time on your next cruise.