Who is responsible?

The main line into my home from the Cox cable/internet box outside my home apparently has a short.  Why is Cox not responsible for fixing this?  I had Cox out to my home and they keep telling me that it is my responsibility to fix.  Everything I have ever known and everyone I speak with tells me that this is Cox's responsibility to fix since this is the line into the home.  I can understand if the lines inside my home were bad, but this line connects directly to the Cox system, and I have not access to it.