Yes you can call through your phone if yourself and other person you are calling has these 2 things, assuming you are not at a port.
1. You'll need a phone that can do wifi calling, I know it is pretty common now, or have an app such a WhatsApp or skype.
2. You must be connected to the wifi which does require purchase of an internet plan to use on board.
Personally I haven't ever had a need to call someone one boat i usually will run into them eventually. I have seen people do it and I'll be doing facetime on my star class cruise to rub it in my sister's face that she didn't want to come.