In a normal telephone wire red-green is line one, yellow-black is line two. In an alarm telephone wire red-green is the incoming telephone line coming from the telephone network interface, yellow-black is the return that provides dial tone to all the telephones in the house. This setup provides the alarm panel with the ability to "seize" the line in case of an emergency to ensure that the alarm can communicate to the central station. The 31-X jack is simply a quick disconnect to allow the home owner to restore their phone service if the alarm has malfunctioned.