Whenever I try to use FaceTime to contact a certain person, it immediately disconnects with the message "X is not available for FaceTime". We are both using a MacBook running Mavericks, not iOS. The same message appears if the other person tries to contact me via FaceTime.
I am able to use iMessage with no problems to contact them, and we have both made and received calls to other people.
Edit: I meant FaceTime calls, yes. I do not know if the other person has used FaceTime successfully with others; by "both" I meant that I have been able to make FaceTime calls and also receive them.