Enterprise Rent-A-Car

I picked up my car in Durango Co. I was surprised at the lack of customer service. They handed me my paperwork and keys and told me to go find the National lot #7. They did not walk to the car with me and do an inspection so that we would both be aware of any cosmetic damages like I've had happen every other time I've picked up a car. I noticed what looked like a white spot that was down by the wipers but didn't think much of it because the car really wasn't very clean. I had to clean up the inside at my house before I used the car. They were busy and sold out of rentals so I didn't complain. I returned the car on time and took the keys into the person at the counter. A few hours later I receive a call stating that they are going to submit a claim for a damaged windshield. I told the man that I had not caused the damage and let him know that no one had done an inspection with me and that I noticed a mark or something that looked like a white spot on it but he didn't care to listen to me. He gave me the wrong phone number to speak to the damage dept and I had to call back. He gave me another number that I can't get anyone to answer. Yesterday I received the emailed bill and I'm being charged $500 for the damage that I did not cause! I am so unhappy with this situation and the customer service that is lacking. I would really like to get some assistance with this issue.