Hi Idahla,
My boyfriend went to OCS a year ago, but I don't think it's changed much.
They get to make very short phone calls (about 15 minutes) on Sundays until the class secures (usually week 4 or 6). Once they secure and get liberty, they can call more often. They might be able to make short calls during the week at that point, and they can find a pay phone on or off base on the weekends and talk for longer than 15 minutes.
Hope that helps!
Kathy