Basic information: Your service professional will need to know where the equipment is, be able to gain access to it, and may require additional information such as the ID for the bad circuits, whether or not anyone has recently moved, added, changed, or reconfigured your telephone equipment and a timetable of symptoms leading up to the failure.

In resolving phone failures, usually a person assigned to administer the PBX will be able to examine logs and other indicators to determine what the failure is. Both the cause, and scope of the failure must be determined.

Outside or inside wiring: The most basic question is simply whether the problem exists with the outside wiring (the telephone company's responsibility) or with the inside wiring.

If the problem is with the outside wiring, this requires calling the phone company that owns the circuit, and identifying the circuit by an ID provided when the circuit was installed. It's an excellent idea to have the circuit ID written down and easily accessible. The phone company doesn't keep records of circuit IDs. But it can't identify the circuit without the ID. If the number can't be easily found, tracking it down becomes a difficult, lengthy and expensive task.

-- Tips courtesy of HomeAdvisor.com