FaceTime unable to call or receive calls

Actually, the problem is with my mother and my sister. Onde of them uses a MBP 2009, the other uses a MBP 2010. The both run Yosemite, but the problem is previous to the upgrade.

Every single time I tried to set up FaceTime to them, it didn't work. They do make calls, but these never actually reach the receiver. I tested it by calling my own MBP, beside theirs. Also, if I try to call them using my computer, the same happens: it calls and calls, but they never receive the call. I know the problem is not in my computer because I can FaceTime with my father with no problems at all. Ideas?