Billed for service that was cancelled in 2007 - but I can't speak to anyone directly about a refund?

Dear Business Forum:

• My small business has multiple 800 numbers.  Because we are billed on several different account numbers, the entire "big picture" is very confusing.
• On February 7, 2018 I received a collection notice that we owed $1893.22 and that we needed to pay immediately.  I called and spoke with Customer Care, who verified that there had been no activity since 2007 on the account number provided in the letter.
• Upon checking our own records, we found that we have not received a bill on this account number for several years.
• This helpful rep issued a cancellation on the account - great.  She also put in a request for a refund, supposedly for the 10+ years that we had been billed.
• Today, we received a bill showing a credit balance of $724.05.  I called Customer Care again to find out why the refund was so small.  We are owed quite a bit more than that.
• The rep today told me that there is a "statute of limitations" that says that credits may only be issued going back 24 months.  Funny, the first rep never said anything like that.
• I said that I would like copies of these bills for the past few years so that I can see what we've been billed for.  The rep said that all that could be done is to submit a request to the "Legal Review Team" and see what they say.  When I asked for contact information for the Legal Review Team, I was told that they "don't take incoming calls."
• When I asked for copies of those past bills, I was told that "They are all going to look the same since you have been billed the same amount every month.
  
  

This entire situation makes me very uncomfortable.  I've been charged for YEARS for service that was cancelled and not used.  When I ask for information, I'm told it isn't available.  If I am to work this out and figure out what is actually due us, I can't do so without the proper documentation AND the right to speak or correspond with someone in the department that will make a decision.

I would like very much to:

1. Get copies of bills dating back to the cancellation date so that I know what I'm dealing with. How can anyone determine the refund amount without documentation?
2. Have contact information for someone who will actually be responsible for determining the amount of the refund due - not a Customer Care rep working from overseas. (The overseas part doesn't bother me.  What bothers me is that they can't make decisions.)
3. Speak with an actual AT & T representative (not a contractor) who can do an overall account review and determine if we are being charged for other numbers we have previously cancelled.
4. Get a copy of this "Statute of Limitations" that says that erroneous charges can only be refunded for a 24 month period.

Please help.  Thank you in advance for your consideration.