Awaiting Paid up letter

We have been paying an account that dated back to 1999 we started paying in 2010 and since then we have paid the account in full. I requested receipts of proof to say we purchased something so long ago and to date we are still waiting. I have requested a letter confirming the account has been paid up in full, since January 2015 as I still haven't received anything. And it is also impossible to get through to the call center, to follow up on where my letter is. Very poor service.