Since 1982, the maintenance and repair of telephone wiring and equipment inside a customer's building has been the responsibility of the customer.  That means all of the telephone wires and telephone equipment on the customer’s side of the demarcation point, where the telephone wires enter the house, apartment or business, are no longer the responsibility of the local telephone company.

If you call the telephone company to come repair your telephone and the company finds that the problem is on your side of the Network Interface Device (NID), you will be charged for the service call whether you choose to have the company do any repairs or not.

So how does a customer know whether the problem is with his wiring or the company's?  The NID is designed so the customer can determine whether the problem is with his lines or the company's.  Check out http://www.homephonewiring.com/ for a layman's guide to home phone wiring.