A Process of Elimination
As is often the case troubleshooting a telephone fault can be a complex business. A fault can be caused by any of the myriad pieces of equipment that make up a telephone system, from the telephone itself, through your PBX and down to the telephone line provided by your telco. Finding precisely which component is at fault is a complex job. With a systematic approach to troubleshooting your fault finding can be made much easier.
The first thing you need to do is make sure your telephone isn't faulty. The quickest way to check the functionality of your phone is to swap it out for a known working one. If the fault disappears then it's a pretty good bet that your telephone is faulty.
If the fault persists then it cannot be the telephone itself that is causing the problem. Ideally you will have a cable tester available in order to eliminate the cabling between your telephone outlet and the PBX. Most modern testers are capable of testing both traditional two and four wire cables and CAT5 cabling present in most modern structured cabling systems. If you don't have a tester, moving your telephone to a different extension would help eliminate the cabling as the cause of the fault.
If the cabling is ok, you need to check the telephone line provided by your telco. That's where a telephone tester comes into its own.
Cable testers come in a wide variety of forms suitable for many different tasks, from simple continuity tester all the way through to highly complex cable certifers.
A continuity tester is probably the oldest style of cable tester. Continuity testers work by putting an electrical signal onto the cable and sending it to a remote at the opposite end of the cable. By placing an electrical signal onto the cable, they are able to detect that the signal was received by the remote correctly. They are typically fairly cheap and are able to detect cables with shorts or breaks.
Qualification testers are probably a little overkill if you are just wanting to troubleshoot telephone problems, as their prime aim is to qualify structured cabling for the use as a medium for carrying high speed network traffic. In order to qualify a cable for network operation, they typically send network traffic down the cable (called the BERT test) to an intelligent remote. The remote is then able to validate the network traffic as being received correctly. If the traffic is received correctly, then the cable is assumed to be qualified to successfully carry network traffic.
Cable certifiers are the most expensive type of cable testers and are typically only used by cable installers wishing to certify a cabling installation.
Once you're sure it is the telephone line at fault, you need a tool to help identify where the problem lies. A basic telephone testing tool, often called a 'butt set', will connect up to the telephone jack and check the line for voltage and a dial tone. You can then dial external numbers to troubleshoot any issues.
More sophisticated testers can sense over or under voltage, show dialled and incoming numbers, check current, polarity and other characteristics of the line. If you need to test modern ADSL lines make sure that you get a tester than can operate on these newer systems. If you need to troubleshoot the ADSL functionality of your telephone line, then a dedicated ADSL tester would be very helpful.
Once you are sure there is a problem with the telephone line you need to call your telecom company or line provider. If there is a problem down stream they are the only people who can solve it. If you are armed with the readings from a tester you are likely to get a more positive response from your telecom provider.
A typical phone tester will also be supplied with crocodile clips enabling you to test up to unterminated (bare wire) installation. If you plan to use a tester a lot it may be worth going for one with memory functions to record commonly used numbers, last number redial and other time saving features, and a headset for hands free operation (a very useful feature if you have to work in a noisy or cramped space, which is most of the time in comms rooms and wiring closets).