I let my friend use my credit for a phone

I let my friend use my credit to get a cell phone 3 years ago, and two years later I start getting mail from a debt collection agency saying that money is owed. I left it up to being a mistake since my friend said that he canceled the service. Over that time I had credit cards closed on me and credit limit lowered. So now I'm trying to settle this matter, but my question is: is there any way to dispute my credit score being lowered with the credit bureau? Or am I just going to have to rebuild my credit little by little?