Being harrassed by collection agencies

Back in Oct 2015, I called and inquired about a POTS line but not to have it installed because of the install cost. Next thing I know I am getting bills from att, I contacted them and they apologized cancelled the bill of $106. A couple of months later I received another bill for $26, I called again and they did the same song and dance with apologizing and cancelled out that bill. Then about 6 months after I started receiving letters from collections agencies about this freaking bogus $26 bill. I explained to them and contacted ATT for a dispute. I contact att as well and caught it,  trying to get to the right department. countless of hang up, countless of ppl not knowing what to do. Then I finally speak someone that claims that they have it taken care of. BUT APPARENTLY NOT now I am getting calls from another collections agency name ERC. I this has became a nightmare and very frustrating. I really feel the agent I inquired with originally is responsible for this mess, and the managers that so called straightened it out. If I was considering ATT for home services this experience has certainly left a bad taste in my mouth.What number can I reach to speak with the correct department to get this taken care of finally.