If I am claiming my personal cell phone for work related expenses, how do I determine the expense amount? I have two other lines on my account as well.

[Cpthowdy]

Would I just use the amount of my phone line?

[MinhT1]

It depends.

If you are employee then your phone usage for work is not deductible.

Up to tax year 2017, job-related expenses were deductible as an itemized deduction subject to the 2% of AGI floor.

Under the Tax Cuts and Jobs Act, the itemized deductions for job-related expenses subject to the 2% of AGI floor are suspended for expenses incurred after December 31, 2017 through 2025.

If you are self-employed, then you can deduct your phone usage. You should deduct the business portion of your phone line only.