responsible for stolen cell phone charges?

I recently had my cell phone stolen from my car. Because I rarely ever used it, it was about a month before I noticed it missing. I contacted the phone company to report it stolen. Three customer service reps and a few weeks later, they told me I needed to file a police report and fax it to them so they could review the charges, which I did. Now the have come back and said I am responsible for all of the charges and are going to turn my account over to a collection agency because I failed to report it expediently. Am I responsible for these charges and how do I clear this up?