Telephone service or equivalent two-way communication facilities shall be provided in all mines at least one of which shall be in service at all times as follows:
(a) A telephone or equivalent two-way communication facility shall be located on the surface within five hundred feet of all main portals, and shall be installed either in a building or in a box-like structure designed to protect the facilities from damage by inclement weather. At least one of these communication facilities shall be at a location where a responsible person who is always on duty when miners are underground can hear the facility and respond immediately in the event of an emergency. "Two-way communication facility" shall mean a system maintained to allow voice contact to come in and out of the working section at all times.
(b) (1) Telephones or equivalent two-way communication facilities provided at each working section shall be located not more than five hundred feet outby the last open crosscut and not more than eight hundred feet from the farthest point of penetration of the working places on such section.
(2) The incoming communication signal shall activate an audible alarm, distinguishable from the surrounding noise level, or a visual alarm that can be seen by a miner regularly employed on the working section.