The Demarc (abbreviation for demarcation point) is the gray box on the outside of your house where the phone line(s) connect to the wiring that is inside the house.
When you’re having phone troubles such as no dial-tone, or noises such as static, you can determine where the problem might be coming from yourself by checking your demarc.
Checking the demarc yourself will also save time in resolving the problem and may also help to avoid any charges that could apply for having a technician dispatched to your home when the problem is actually caused by something inside the house.
Find the demarc box on the outside of your house (usually in the back yard or on the side of the house), and using a flat tip screwdriver open the cover by loosening the screw on the side of the cover.
With the cover open, you should be able to see the module that connects to the inside wiring from the house.
You can disconnect the inside wiring by unplugging the short phone line from the side of the module. This is also how you “reset” the phone lines (by leaving unplugged for 5 minutes or more).
Connect a standard corded telephone to the phone jack on the module and test the line (by checking for dial-tone and/or listening for noises while making a call) from this point to eliminate any problems that may be caused by the wiring or a malfunctioning device inside the house.
If the problem has cleared up when checked from this point, then the cause may be the wiring inside the house or possibly a malfunctioning phone or device connected to the phone lines such as an alarm system.