I have used the AT&T universal card, but they will charge you a $7.50 fee just to connect you, even if all you get is the answering machine!
My suggestion is to buy pre-paid calling cards locally when you arrive in a port.
If this is too confusing (and it can be!) then one thing we have done in big cities before is to go into hotels and ask them if they have a business center. Many times they will let you call internationally for a reasonable fee. It isn't cheap, but at least you don't get ripped off.
Sorry, I know this is sketchy advice, but it is the best I can do for now.