47 CFR 80.103 - Digital selective calling (DSC) operating procedures.
(c) DSC acknowledgment of DSC distress and safety calls must be made by designated coast stations and such acknowledgment must be in accordance with procedures contained in ITU-R M.541-9 (incorporated by reference, see § 80.7). Nondesignated public and private coast stations must follow the guidance provided for ship stations in ITU-R M.541-9 (incorporated by reference, see § 80.7), with respect to DSC “Acknowledgment of distress calls” and “Distress relays.” (See subpart W of this part.)
(d) Group calls to vessels under the common control of a single entity are authorized. A group call identity may be created from an MMSI ending in a zero, assigned to this single entity, by deleting the trailing zero and adding a leading zero to the identity.