In order for a phone line to function properly, it needs to be an independent circuit that does not touch any other phone lines. Should one phone line come in contact with another phone line, both phone lines will stop functioning properly. You can determine if your phone lines are touching by using the "Continuity" setting on your digital multimeter at the network interface device installed on the exterior wall of your house.
Disconnect anything connected to a telephone jack in the house. You need the telephone lines to be open in order to test them.
Locate the network interface device box, or NID, for your phone service. It is a large grey box with the words "Telephone Network Interface" printed on it and is usually installed by your telephone company on one of the exterior walls of your house near your electrical meter. This box is where your phone lines connect to your telephone company's wiring loop, so it must be located somewhere near or on your property. If you cannot find your NID, contact your telephone service provider for its exact location.
Unscrew the screws labeled "Customer Access" on the NID's customer access cover with a screw driver and remove the cover. Any screws that are not labeled "Customer Access" can only be removed by the telephone company that installed the NID.
Disconnect the phone lines that you want to test from their NID test jacks. Once disconnected, the phone lines will be completely open and ready for testing. Wait one minute with the phone lines disconnected before proceeding to allow any electricity in the lines to dissipate.
Set the digital multimeter to its continuity setting. This setting is usually marked by a symbol that resembles a sound wave, but see the documentation that came with your device for specific instructions.
Touch the tips of the digital multimeter's leads together. If the multimeter is functioning properly, a reading will appear on its digital screen and it will emit a beeping noise.
Connect one of the digital multimeter's leads to one phone wire and connect the second lead to another phone wire. If the multimeter does not detect any continuity, the phone lines are not touching. If it does detect continuity, the phone lines are touching somewhere and will not function properly.
Repeat the test for each pair of phone lines. Once every pair has been tested, you will know which phone lines work and which phone lines do not.
Things You'll Need