Someone else gets my FaceTime calls -- strange problem

Problem:

Occasionally when someone tries to FaceTime me they get someone else (but the same someone else every time)

I'm based in the UK but this person is in the US. My best guess is that they have gotten one of the old phone numbers I used while temporarily visiting the US that must have been associated with my Apple ID at some point.

Is there a way to find all the numbers associated with my ID and remove them? No foreign numbers show up in my iMessages preferences and I cannot find anything in http://ift.tt/k8WFVt

Any suggestions? (It's also possible that I have mis-diagnosed the problem)