To make your phone work as a card reader, you will have to install an online billing app. There are quite a few in the market that you can use. Apple Pay for iPhone, Google Pay for Android phones, or the new and popular ScanPay are great apps for safe mobile payments.
You can qualify for a cell phone tax deduction from cell phone charges incurred when the mobile phone is being used exclusively for business. There is not an IRS cell phone deduction for self employed people, exclusively.