Hardware with speech output allows the speech volume to be varied over a range of 18 dB or more, and where the control works in steps, at least one intermediate step gives 12 dB of gain above the lowest setting; handsets and headsets meeting ANSI/TIA-4965 are treated as compliant.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.