In finding creative ways to collect taxes and burden small business with onerous accounting / tracking / paperwork, the IRS wants to tax the personal usage of business cell phones. They already do this on company cars used for personal activities.
From a TMCNET article:
Specifically, the IRS has started to implement regulations for identifying personal versus business usage on company cell phones. ... "To be able to exclude the use by an employee from taxable income from an employer-owned cell phone, the employer must have some method to require the employee to keep records that distinguish business from personal phone charges. If the telephone is used exclusively for business, all use is excludable from income (as a working condition fringe benefit). The amount that represents personal use is included in the wages of the employee. This includes individual personal calls, as well as a pro rata share of monthly service charges." [IRS]
"While these regulations are only in their infancy phase at this time and only applicable to government agencies, it is only a matter of time before these regulations become a requirement for all businesses."