That is a factual dispute which requires a fact finding (hearing) if you are contesting the citation. Contact a local criminal defense attorney who specializes in traffic offenses. Good luck.
Depending on circumstances, and on what the officer put in his or her report, you might have a defense. There are many good traffic ticket attorneys in your area who will likely offer a free consultation. Talk to a couple of them. Good luck!
The citation must properly cite the RCW/municipal code of the violation. If the officer's affidavit is not sufficient to prove cell phone use, then your ticket will be dismissed.