Can Send but Cannot Receive FaceTime calls

Hi

I have a 2011 Mac Mini OSX 10.11 and I can dial mine (and others) iPhone/iPad etc using my Mac however cannot receive calls. It seems to be dialling ok from the iPhone however there is no indication on the Mac that there is an incoming call even with the FaceTime app open

Any ideas?

I have been having these issues for a while now, well before I upgraded to El Capitan.

Thanks