Once a phone call is initiated it poses little or no risk as it continues. If I start a phone call while I'm at a stop light and continue with it I'm really not posing any additional danger to anyone. By comparison taking your eyes off the road to read and write a text message is inherently dangerous any time you are attempting to drive while doing so.
Ironically, the initial justification for cell phone bans (before the era of smartphones) was the exact opposite. Cell phones with physical buttons were easy to dial with, could be done by touch many times, and scrolling a linear list of contacts was easy. The "concern" was that an emotional call (or any call at all) would be distracting in a way that listening to the radio wasn't. This is why other "fiddling with things" tech, even available then, wasn't included in the ban here in CA. (I'm talking about things like dedicated GPS receivers, and classic iPods.)
Text messaging alone didn't seem to really take off until the early-mid 2000s in the US outside of certain circles.
Nowadays, of course, it's almost completely the opposite. Phone calls are rarer, and your mobile device is multifunction and used for all sorts of things by the broad populace on a regular basis.
I have to generally agree with the grandparent/first post though. If EVERYONE is doing it regardless, the draconian law should be changed so something more meaningful and fairer to enforce. I'd say more people are breaking the cell phone law than are breaking speed limits -- and that's something. In CA you're not allowed to have your cell in hand even stopped at a traffic light which (as annoying as it is to have to honk at someone that the light's changed) not as much harm as that ticket would seem to imply.
My suggestion: "special circumstances" for injury accidents and reckless driving / moving violation crimes with hands off the wheel and a renewed emphasis on "Don't do distracted driving" vs "Don't look at your cell phone". Let's be realistic, and we'll have much more compliance.