At&t Extorted with ruining my credit for $500 cable boxs that were returned but claim were not.

I have been an att cell phone customer for 10 years. NEVER made a late payment. I also had Uverse for a period of time and moved to an area that did not have Uverse so I had to discontinue service. I reveived a ups label from att and sent in the equiptment. 6 months later I start getting phone calls from a bill collector for $500 for equiptment that they claom was never received. My credit score was also at the same time RUINED from a 760 to a 600. ATT said the only way they could help me was If I paid the $500. I refused at that time and took up the claim with the credit agancys and they sided with me removing the negative mark on my credit report. A year later the debt was sold to a new collector who submitted the negative mark to the credit agencies. AT this point I dont have anymore time to deal with it and was forced by ATT into paying the $500. I was extorted and robbed of my time , my credit and my money by att.