Debt Collections on my credit report from DirectTV/ATT Internet!

I had an ATT/Direct TV Combo account 3yrs ago and canceled it due to a bad service area. Nothing wrong otherwise. I was told at the time of cancelation all I needed to do was turn in my equipment and there would be no additional charges due to the inconvenience. Fast forward 1yr, a debt collector was contacting me and also hit my credit stating that I owed $600 plus to ATT. I called and thought that it was taken care of as it came off my credit. Fast forward another 1 1/2yrs and another debt collector is now contacting me and now trying to collect $650 and again my credit has been hit. This is ridiculous! I cannot get ahold of anyone from ATT and have been passed around to different reps each telling me its either not their department or they cannot find the collection. This is severely affecting my credit and I want it removed. Who can help me with this???