People can call me via FaceTime, but certain callers get the "not available" message when they FaceTime me

I have FaceTime on my iPhone, iPad and iMacs, and it seems to work flawlessly, most people have no trouble calling me. However a couple of people have tried reaching me via FaceTime and they report that they get the "User not available" message.

Why would most people be able to FaceTime me with no problem while a couple of people would get the "User not available" message?