Its very expensive, best bet is to wait to get into a port and then use the phone,
or find an internet cafe and send an e-mail. We've found these nice cafe's all over the caribbean, and at a resonable price too.
Our son had to call us when we were on the Elation (due to a death in the family). The charge to our sign and sail card was $21.00. I do have to say, that in an emergency, it was wonderful to be able to phone home. I wouldn't plan on using the phones to chat on though. Bon Voyage.
It's around $15 per minute to call from a ship. The reason it's so high is that the ship uses satellite time for calls. Do what the crew does -- run to a pay phone on the pier and call with your calling card. You'll find public phones everywhere.