United recently started announcing at the beginning of flights that using your phone with sound on the speaker is prohibited, as a new official policy.

Talking on your phone has always been prohibited while in flight. Having high speed internet on a plane is not new.

I share your concern. Airlines seem to be anticipating this. There was a recent publicized incident of American Airlines removing a woman from a flight for playing audio over her phone speakers. United has similar policies. As I understand it, both are saying they will ban passengers over it.