You wouldn’t want your anesthetist or surgeon yammering or texting on their phone while doing your open heart surgery, and you wouldn’t want your pilot doing it while your plane is ready for takeoff.
Uh, pretty sure the cockpit crew will acknowledge being cleared for takeoff via 2-way radio communication with tower just before takeoff. Granted, in a FAR-121 airplane, that’ll be the non-flying crewmember, but any plane that has a flight crew of one, that’ll be the pilot.
(Or are radio communications–CB, aircraft radio, etc–okay, and cell phones not? If so, why?)