29 CFR § 1926.1420 - Signals - radio, telephone or other electronic transmission of signals.
(a) The device(s) used to transmit signals must be tested on site before beginning operations to ensure that the signal transmission is effective, clear, and reliable.
(b) Signal transmission must be through a dedicated channel, except:
(1) Multiple cranes/derricks and one or more signal persons may share a dedicated channel for the purpose of coordinating operations.
(2) Where a crane is being operated on or adjacent to railroad tracks, and the actions of the crane operator need to be coordinated with the movement of other equipment or trains on the same or adjacent tracks.
(c) The operator's reception of signals must be by a hands-free system.