If you need a cell phone for work, your employer can insist that you use your own. Many businesses prefer providing staff with company phones. If your employer swings the other way and requires you use your personal phone, the company may reimburse you. If they don't cover your expenses, you may get a tax write-off out of it.
TL;DR (Too Long; Didn't Read)
Employers can insist you use your own cell phone for work, but they may reimburse you. If they don't, look to tax write-offs.
BYOD or COPE?
BYOD or COPE are the two choices for phone use. A company that goes COPE provides "corporate-owned, personally enabled" technology which means they own the phones or the laptops and let staff use them. BYOD is short for "bring your own device," where the company has you make business calls on your own phone.
BYOD is a lot cheaper for a company than setting up a COPE plan. A COPE policy requires a big, complex cell-phone plan covering possibly dozens of employees. Many workers hate doubling up on their phones. COPE does offer better security, as it's easier to monitor tech usage.
If you drive your own car for business, reimbursement is simple: add up the number of work miles and get a per-mile payment. Phones are more complicated. If you have an unlimited plan, for instance, using your phone for work won't add to your costs. Some businesses don't bother with reimbursement for that reason. In California, however, state law says employers have to reimburse part of your phone bill if they require you use your own. It doesn't matter whether your plan is unlimited or not.
If your employer does reimburse you, the reimbursement usually isn't taxable. You shouldn't owe tax provided you record and report the work-related usage and provided the company doesn't pay more than you spent. The IRS doesn't require your employer to calculate exactly how much business use cost you, which makes things easier.
Taking a Write-Off
If you document your on-the-job cell phone expenses, you may be able to claim them as a tax deduction. You take the write-off, including actual work expenses and depreciation and wear and tear as an itemized deduction on Schedule A. Unreimbursed employee expenses are lumped into the "miscellaneous deductions' category along with tax preparation bills and job-search costs. However, you have to deduct 2 percent of your adjusted gross income from your miscellaneous expenses first. Whatever is left after the deduction is what you are allowed to claim.