Wrongful Billing and Credit Score

To my shock, I received an overdue bill of $300+ for the activation and cancellation of an AT&T cellphone line. It was impossible that this account was under my name since I already had an existing account with the company and had never opened a second account. My first thought was fraud, but it turns out that Radioshack had somehow accidentally activated two accounts the day I asked to open one. Fortunately, an AT&T representative said that my dispute for this wrongful billing would likely be successful since it was done without my knowledge and that there had never been any phone use in the second account.

Great. However, I am aware that this overdue bill have negatively affected my credit score, even if it was through no fault of mine. I don't think this will come off my credit report by itself once AT&T takes the bill off under my name, right? The AT&T representative told me to request documentation of proof and send it to the credit bureau (all three of them??) to get it off of my credit report. My mom says that it is not I but AT&T who should directly send the documents to the credit bureau because it was them and Radioshack who are in the wrong. Who is right? What is the proper procedure for me to take if it should be me?