What does "Notice in Writing" means ?

I want to cancel my auto insurance policy.
Does calling my insurance company on the phone to request canceling my policy counts as "writing in notice" ? Or do I have to write a letter and send it by mail to my insurance company stating that I want to cancel my policy ?

Thanks
Update: My company is Esurance.com
They have a 800 # and email contact.

Do I call to speak to a representative, email them or write the letter and send it by mail ?

Thanks
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