Cal. Code Regs. tit. 8, § 7140 – (19-90). Dual Signaling System.

(a) Every shaft in which men are hoisted or lowered shall be provided with a dual electrical system for shaft signaling.
(b) When a dual method of signaling is employed, one shall be a bell system to signal for movement of the shaft conveyance. It shall be used for no other purpose.
The second method shall be used to call for the shaft conveyance and may also be used for other communications, except to signal for movement of the shaft conveyance. This method shall consist of a system of telephones with buzzers or horns loud enough to be heard clearly.
(c) (19-92) The control for the signal for movement of the shaft conveyance shall be located at the shaft, within easy reach of a person in the shaft conveyance.
(d) The control for the signal used to call for the shaft conveyance shall be in a convenient location well away from the shaft.
(e) (19-10) Hoist controls shall be placed or hosed so that the noise from machinery or other sources will not prevent hoist men from hearing signals.