Indeed not, but far more people are using electronic messaging of whatever kind instead of actually speaking to people, and a smartphone's "keyboard" is silent (unless you have the key click on, and even then it's not loud).
My experience of quiet coach "misuse" is more based around the playing out loud of films and TV shows on tablets and phones than on phone calls.
The idea that this crap is acceptable in any part of the train is my main problem with "quiet zones" it gives inconsiderate tossers the idea that if they are not in a designated quiet area they can be as noisy as they like.
Playing music/films is not on anywhere on public transport and I would also put loud lengthy phone conversations in that category.
Quiet coach should be just that, QUIET: no un-necessary noise, including groups talking, kids playing, ANY phone use, leaky headphones, irritating keyboard clicks, etc.
As it is typically only 1 out of 6-8 std class coaches it is far from unreasonable for it to be restricted to those who want to travel in peace.
In my experience the vast majority of passengers on IC routes make little or no noise, it is only a minority on most services (****ed-up late weekends excepted) who are creating most of the disturbances.
I would prefer the default position on IC services to be no loud noises, noise generating electronics or phone converstions and for selected Loud Carriages to be designated for those who do wish to be noisy, perhaps call them "social carriages" or something.