If the personal phone runs $50 a month and you have unlimited calling and it is not a separate business line then the IRS could say using it for business did not increase the bill over the personal use cost so nothing is deductible. This harkens back to the old rules for land lines. However if you upgrade to a higher program to now include more minutes, unlimited minutes or texting or other extra functions then the extra cost up and over the base fee would be a business expense.
