Okay, I understand that I'm not capable of driving while I'm holding a phone. And of course, I am capable of driving if the phone is not against my ear (wireless or blue toothed, or whatever). Yes, that was sarcasm.
However, if an officer issues a citation because he or she sees the driver holding a phone against their ear, how do they know that the driver was using it? Maybe they were just holding it against their ear. Maybe they had a (fill in the blank) against their ear. Could've been holding any one of a million things against their ear. Highly unlikely, but entirely possible. Maybe even just to mess with the officer. ;-)
If the burden of proof is on the officer, how do they prove that the driver was actually using the phone?