I think you are smart to question your employer’s request/demand. You could suggest that the office have a separate business phone with the credit card reader for use by employees. But there should be no reason for you to need a credit card reader on your private phone.Wow. Yes. As the owner of the cell phone I'd be held liable. Our clients are people within the community...a small community...and if any of them have issues with their credit card information being stolen I don't want my reputation tarnished. Or if something were to happen and my employer were to blame me...that would be awful.
If a refusal to comply with the employer’s request/demand leads to your job termination, you should apply for unemployment benefits. You might be eligible. For eligibility, you can check out Pennsylvania’s Office of Unemployment Compensation: https://www.uc.pa.gov/unemployment-benefits/Pages/default.aspx