Calvary Portfolio Services and Sprint Outstanding Balance My @#$!! Internet

   I received a letter from Calvary a few days ago saying they bought this account from Sprint and that I owe $161.52.  I haven't been a sprint customer for at least 5 years and I paid my account in full before I closed it.  So, my first question is....WTF?!  Second, Why am I just hearing about this now.....5 years later?  Now I see this on my credit report as a "potentially negative".  Which helps explain why I've had such a hard time getting a car loan lately.  Next I called Calvary, and a young lady wanted to verify who I am by the last 4 of my social.  So she called out 4 digits that are not my social.  But she new my last three addresses and my phone number.  How can this be on my credit report?  She couldn't answer that as well.  (I did not give her my social # by the way)
  I disputed the account on Experian and Transunion.  I also told the young lady at Calvary that I will file with the BBB and she said she would label the account as being disputed. 
  Not sure what step to take next.  Its not a lot of money, but I refuse to pay it because it's not my debt.
What do you all think?