Can I deduct half of my personal cell phone service since I used it for my small business half the time?

[robtm]

If you've got the records to show you used your phone for business purposes, then you can deduct as a business expense the time used, If you do not have the records the IRS probably would not allow the deduction. The burden of proof would be on you to show it was a legitimate expense. 

[MonikaK1]

If you're self-employed and you use your cellphone for business, you can claim the business use of your phone as a tax deduction. If 30 percent of your time on the phone is spent on business, you could legitimately deduct 30 percent of your phone bill. You can also deduct depreciation on the cost of the phone itself. 

See this TurboTax tips article and this IRS news release for more information.

[Opus 17]

There are two arguments.  One argument is that if you have a flat rate plan (most people do) then you can't claim anything because your business use does not increase your cost over what you would already pay for personal use.  The other argument is that you can deduct the percentage of use that you can allocate to business use by some kind of reliable record.  (I think the first argument is more logical but there are indications that the IRS will accept the second argument.)

The problem is reliable proof.  The IRS considers proof to be reliable if is it written down (or entered into a computerized record, spreadsheet or logbook) in a contemporaneous manner, that is, close in time to the occurrence of the expense.  It's not enough to guess in March 2023, that your business use in 2022 was "about 50%".   You would need some kind of reliable and contemporaneous records, like number of calls per month divided into business and personal, number of emails and texts, minutes of use, or something similar.

If audited, you are not entitled to any deduction or expense that you can't adequately prove with reliable records.