How do I determine the business use of my cell phone as a business deduction? Is it a percentage?

Yes, it is a percentage but only if you are self employed. 

Thank you.

Can you be more specific. Percentage of what exactly?

That’s the idea. Monitor your usage over what you would consider usual usage over a 3-4 month period. Use that percentage to apply to your yearly cost when you do your taxes. 

Yes, that will work.   keep detailed records with your tax paperwork.