Well, the officer did state that he wasn't sure what you were doing and he only speculated that it could have been that you were on your cell phone.
Not sure if it will matter that you can prove you were not. It is going to come down to what he observed your vehicle doing for him to state that it was careless driving on your part.
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39:4-97 Careless driving
39:4-97. A person who drives a vehicle carelessly, or without due caution and circumspection, in a manner so as to endanger, or be likely to endanger, a person or property, shall be guilty of careless driving.

