They can call you at 888-DC-AT-SEA. They'll need to know the name of the ship you're on and your cabin number.
The cost for a call in either direction is around $7 or $8 per minute billed to your stateroom account.
Cell phone coverage on the Islands depends on your carrier. If you're on the 3 or 4 day cruise in the Bahamas, it's pretty difficult to use a cell phone because of the massive amounts of fraud. I use AT&T Digital One Rate and they just plain don't provide service there at all, even though I have International Calling enabled.
Very often, there are pay phones set up near the docks where the cruise ships tie up so that might be the easiest and most reliable method to use.
Others find ot very convenient to use the Internet Cafe and to keep in touch using web based email.
hope that helps