I have heard from other parents that they used Skype a free or very inexpensive service using Mids laptop to make calls (including live video of person you're talking to) to your computer. I think you can also use for just phone calls if you don't have computer. Look into that. we've used it to talk to DS and works well. good luck!
My Captain husband says DON'T buy a calling card in the US. They can be purchased very cheap in whatever country they visit. Plus between email, SKYPE, and (if the Captain is nice enough ) sometimes cadets are allowed a call from the ship. My husband allows his cadets a 5 min. call as a reward for good work I think once per 35 day trip. Also the ship's slop chest may have calling cards available for purchase (about $35 on an MLL ship) that allows phone calls from the ship. There are going to be many ways for your cadet to call or contact, just keep your cell phone on and ready as the call can come day or night!
Keep in mind that your mid is in good hands and no news is always good news.