Cell phones are "listed property" and special rules apply. Listed property are certain items that have common dual use (personal and business) and have been identified by the IRS as frequently abused deductions. These include cameras, computers, and cell phones. One of the requirements is that to take any depreciation, you must use the property at least 50% for business and be able to document that fact with logs or other records. Assuming you can document 90% use, you can take depreciation for 90% of the value.
That said, you are still eligible for two different ways of expensing the cost in one year instead of depreciating over 5 years, the $2500 safe harbor, or section 179. But because this is listed property, you should start by listing it as an asset, and there should be a specific category for listed property. Then if you are eligible for section 179 or the safe harbor, turbotax will offer them to you when you are finished with the asset interview.
