I'm not a tax expert at all, but I disagree with the statement that a cell phone is considered "listed property" by the IRS. And that particular change (effective 12/31/2009) precipitated a handful of other IRS interpretations that significantly changed/simplified how cell phones are handled for tax purposes by both employers and employees.
The "OP as sole proprietor" may require some further analysis, though. I encourage readers interested in this topic to review the following article:
https://www.burr.com/blogs/tax-law-insights/2011/10/07/irs-rules-for-employer-provided-telephones/
