A junction box is a connection point used in supplying telephone service to various points in a building. Providing an interface between exterior and interior wiring, junction boxes contain terminals for each of the four colored phone wires and allow for expansion of a phone system.
Phone lines from a utility pole or underground feed are contained in an exterior box called the Network Interface Device. From the NID, wires enter a building to a point in a basement or utility room where a junction box is found. From a junction box, interior wires travel to connectors found on walls and baseboards for phones and other devices.