Demarc is the point where the telephone company's responsibility ends for the line and where the customer's begins. This is also referred to as a NID or Network Interface Device. It usually has an orange cover on it for indoor installations but can be pretty much anything if the lines come in and are terminated outside. Yes, you can flip the CO cards (they have white handles) to see if the problem follows. To swap their physical system responsiblity for port assignments, that requires more intense work. This is more of a technician-level requirement.

Ed Vaughn, MBSWWYPBX