Debt collectors calling and I can not get information on account that has been closed.

First, I would like to thank you for your response to my post. The nearest ATT Store is about 3 hours -one way from my house. I am not able to drive there nor do I not have a cell phone either.  I had another debt collector called after I posted this and she explained how I might go about this.  She gave me the account number she had and told me to ask for the in house billing management. I tried to call ATT back, but their office was closed for the day.  All the equipment was returned via a box that was sent to me by ATT in May 2017.  I only had the unit of about a month and a half.  There was no damage to it when I sent it back to them.  I paid the final billing, gave them a forwarding address, and I did not receive any other notices about this account.  Then last week a debt collector called me and demanded payment.  This week several other debt collectors have called and I called ATT in order to find out what the matter was all about.  As stated I did not have the code or the answer to the personal questions.  Therefore I could not get any information.  So tomorrow I will try again for the 6th time to try and get some information regarding this claim against me. I refuse to pay any money either to the debt collector or to ATT until I get a statement and explanation as to why ATT believes I owe them money.  I do not feel that I am responsible for this claim.  I did everything that the representative, who I contacted to disconnect my service, told me to do.   ATT had my forwarding address and if they did send a billing statement to the old address-the post office would have forwarded it to me as they have my current address on file.