Inappropriate billing

Over the past two and one-half years, I have paid over $1,800 for mobile service (two lines) that I have never used.  The bills were autopaid and not noticed by me, as I have several Xfinity TV and Internet accounts.  When I called to request a refund, I was told they would not provide one because I had requested the service.  I told them that I did not request the service and did not know how my credit card number was on the account — until I realized it was the same credit card used for other Xfinity Services.  Anyone got any ideas on how to get my money back?  Very concerned, as they can clearly determine that I never activiated any phones nor utilized their services.