How To Disconnect Inactive Lines From The Phone Company
Go to the telephone network interface box located on the side of your home. This is easily recognized as it is usually a large flat gray box with wires coming into the bottom. It is also labeled with "Telephone Access" or similar wording. Open the box by unscrewing the bolt (standard screwdriver) on the right side and opening the door. The door has a latch that will make it a bit hard to open, but it will snap open. You will now see the wires that lead into your home connected to screw terminals. Do NOT disconnect ANY wires.
Next to the terminals you will see what appears to be 1 or more short standard telephone cords plugged into a rubber-ringed (RJ-11) jacks. These are the connections to the telephone company's trunk lines. Unplug the small cord to the unused phone lines coming into your home, usually the black & yellow wires or other colors that are different from your main line wires that are usually red & green. (They may be marked with old or strange phone numbers, or they may have no markings at all.) If you have trouble determining which lines are active and inactive, click on "How To Determine Active Phone Lines".
The latest telephone network interface box has a new wire connection system that does not have the small phone cord and RJ-11 jack as mentioned above. This box requires you to physically remove the wires from the snap-down connectors. Merely lift the plastic connectors from the bar and pull the wires out. Be sure to keep all common color wires connected together to maintain the integrity of your home's wiring by stripping, twisting, and taping the "red" connected wires together and then strip, twist, and tape the "green" connected wires together.
You have now disconnected the inactive phone lines in your home from the phone company's trunk. Close the door on the network interface box and resecure the bolt.