Tax Year Prior to 2020: Cell Phone Deduction -- 50% of my cell phone usage is for work. How do I deduct this?

If it is for a job for with you work as an "employee" receiving a W-2, and they are not reimbursed to you, then you can deduct them as an Unreimbursed Employee Business Form 2106.  It goes on Schedule A Itemized Deductions in the Misc section subject to a 2% of Adjusted Gross Income threshold.  If you itemize deductions (as opposed to the Standard Deduction) , and your expenses are high enough, it may benefit you.

Where do I enter job-related employee expenses? (Form 2106)

https://ttlc.intuit.com/replies/4800418

How does the itemized deduction differ from the standard deduction?

https://ttlc.intuit.com/replies/3302313

What's my standard deduction?

https://ttlc.intuit.com/replies/4208574