30 CFR § 23.9
Special requirements for complete devices.
Telephones and signaling devices will be considered nonpermissible if used
under any of the followings conditions:
(a) Without the approval plate, mentioned hereafter.
(b) With unprotected openings in any of the explosion-proof
compartments. This condition refers to any openings in these compartments,
but especially to those equipped with removable covers.
(c) If not complete with all of the parts considered in the
(d) If installed or connected otherwise than in accordance with the
instructions furnished by the manufacturer.
(e) If modified in any manner not authorized by MSHA.