Do everything you can to get this resolved clearly; and make sure you send letters to all the credit reporting agencies when it's resolved. Even then it's not guaranteed this won't come back.
Personal experience:
In 2009 I moved out of a house, I had ATT DSL, called, cancelled the account, and paid the balance on the account over the phone ($20)
6 months later receive a notice from a collection agency about an unpaid bill (for $20+70 in fees and interest) to ATT. Call ATT, after discussing with a rep, and faxing a copy of the bank statement for it, he agrees it was paid, and ATT's records were wrong, I give him the collection agency number and account number to call and correct it. I further call the collection agency, give them the name/extension of the rep at ATT, the account number to reference, and they assure me they will clear this when they confirm the error with ATT.
6 months later, a different collection agency sends me a notice for an unpaid bill (originally $20 and now up to $130) Unfortunately this was shortly after my ex left me; I had closed the formerly joint account, and she took(stole) a file drawer full of records including my records on this. So with no means of proving that this was paid, I decide to just pay off the collection agency rather than have it end up on my credit report.
Jan 2014, an account is put on my credit report as delinquent for $130, by yet another collection agency. Calling them reveals that *surprise* this is in regards to an unpaid bill from ATT in 2009. Further, my credit score fell from 730 to 650 on the back of this one account, causing me to be denied for a rental property I applied for at the time. Even now after having the issue cleared up via the older receipt, and sending letters to the credit agencies, this account is still listed as a negative on my credit which has only recovered to 690, and is costing me nearly 1/2% in interest on my new mortgage.
Just an anecdotal warning about how slimy collection agencies are, and how long they will continue to harass people even after the account has been paid.