30 CFR § 75.1600-1
Communication facilities; main portals; installation requirements.
A telephone or equivalent two-way communication facility shall be located on
the surface within 500 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 men are underground can hear the facility and
respond immediately in the event of an emergency.
[38 FR 29999, Oct. 31, 1973]