Complaint Review: Texas Tollways

I have received a bill from a collections agency, NCO Financial Systems, Inc., saying that my tollway charges from September 2011 have not been paid. I did drive the tollway in September and am sure that that aspect of the billing is correct, however, I have called Texas Tollways multiple times since then and asked if I have any other bills that I owe them. They have repeatedly said "no" and then I get more late notices. In January I called because I had a bill due for $6.00 and I wanted to be sure to pay it all at once if there were any others, so I asked again and they said I did not owe them anything. But now they have sent me to a collections agency for bills from September 2011. I tried calling the collections agency and when I was very specific with when I called and what I was told, she hung up. I wasn't yelling, cussing, raising my voice--just stating the facts. The woman on the end of the line, had asked for my phone number so that she could call me back if the call was dropped. She hasn't called back. I'm not sure how to handle them. For $4.80 in toll bills they are trying to charge me $154.80. I will try calling back and I will not take any more toll roads ever. In my opinion, what they are doing is theft and the state is backing them up on it.