According to your FMCSA article Triplex...
Use of mobile phones is restricted for CMV drivers
This rule restricts a CMV driver from reaching for or holding a mobile phone to conduct a voice communication, as well as dialing by pressing more than a single button. CMV drivers who use a mobile phone while driving can only operate a hands-free phone located in close proximity. In short, the rule prohibits unsafely reaching for a device, holding a mobile phone, or pressing multiple buttons.
How can drivers use a mobile phone and still obey the rules?
Locate the mobile phone so it is operable by the driver while restrained by properly adjusted safety belts.
Utilize an earpiece or the speaker phone function.
Use voice-activated or one-button touch features to initiate, answer, or terminate a call.