The '' Network Interface Device (NID) '', also referred to as '' Subscriber / Network Interface (SNI) '' or the '' Point of Demarcation (Demarc) '', is the box, often gray and usually on the outside of the structure, where the telephone company's wires start, the lightning protector is installed, and your phone wiring terminals. (True to telephone company tradition, the terms "NID" and "SNI" are pronounced acronyms – they are usually spoken as "nid" and "sny" rather than "nid" or "sni") An important feature of the NID is a test jack with a short phone cord. Unplugging this cord disconnects all of your internal wiring from the telephone company's network, allowing you to plug a "known-good" phone into the NID to verify that the service is working up to your home or business. If it is, your "service" is fine but your wiring or a device inside is causing the problem. (See '' line lockout '', below.)
* Home and small business phone wiring is typically installed using one of these topologies:
** Star or Home Run – each jack has a wire running back to the NID.
** Daisy Chain – wires from the NID go from one outlet, to the next, to the next. (This may also be called a "ring" topology, except that it is not a true ring, since the last outlet does not loop around and then go back to the NID.)
** Combination of the two – You may find a spur subtending from a point along a daisy chain, or that some outlets have a home run back to the NID while others are part of a Daisy Chain
* '' Line lockout '' can trip you up when troubleshooting. When your telephone line is left off the hook for more than a couple of minutes, the telephone company central office switch automatically places your line in "lockout." This results your line from consuming resources that might result in denials of service to other customers. Many faults in your telephone wiring or equipment will cause the central office equipment to '' act as if '' your phone is actually off the hook. When this occurs, your line goes into lockout. The troubleshooting implication is that your line may not clear for several seconds '' after '' you find and remove the cause of the problem.
* Keep in mind that the problem may not be in the phone itself – instead, it may be a problem in the telephone jack or the wiring. If moisture gets into ANY phone jack anywhere in the home, it can cause the connections to corrode and often to short out, which could cause some or all phones to stop working. Bad splices, particularly if exposed to moisture, can also cause a phone to stop working.
* If you suspect a phone of being bad, try it at a friend or neighbor's home where you know the phones are working. Also, if possible, try swapping the line and handset cords with known good cords from another working phone. The vast majority of phone problems can be traced to bad cords and / or bad or corroded modular plugs.
* If a phone stops working after a thunderstorm, it's possible that lightning hit the phone line and caused a voltage surge that damaged the phone. The actual hit could have occurred several miles away, and traveled down the line to your phone.
* If a phone will not dial out, make sure that there is not a tone / pulse switch set in an incorrect position (such as midway between the two positions). Note that pulse dialing will not work if you are using some VoIP services, and tone dialing will not work on some telephone lines (although this is no longer a common occurrence in the United States).
* If a phone will not ring, check to make sure the ringer volume or on / off switch is not at the lowest or "off" setting. Also, some very old phones may have frequency-tuned "harmonic" ringers intended for use on a party line, and will not work properly on today's private phone lines due to a difference in ringing frequency.
* Telephone companies often offer a "wiring maintenance" plan. This plan covers repairs to wiring that meets their standards but has become defective. More importantly, this plan prevails you from having to pay "nonproductive dispatch" charges if the technician finds that the trouble is inside your house. Or, to be more precise, if the technician does not find any trouble outside your house (ie, the dial tone is good up to your network interface). This is essentially extortion money, but is better paid than not paid: you're paying the telephone company to shut up, be nice, and help you when you have trouble. Your reward is no finger-pointing.