Sprint Requested service cancelation, requested ignored, sent to collections Nationwide, Nationwide

I admit, the beginning of this is my own fault.  I neglected/forgot to cancel my Sprint account, and the payment on the balance because overdue.  When I realized this in Jan. 2011 I contacted Sprint explaining that I no longer needed their services, and requested a final bill be mailed to me.  I was aware the payment was overdue due to my own neglect in canceling in a timely manner, and had no objection to paying the outstanding balance of $334.41 in full once a final bill noting the account had been canceled was received.  Time past, and Sprint never sent me a bill.  Instead of canceling my account, and mailing me bill they sent my account to a collection agency by the name of RECEIVABLES PERFORMANCE MANAGE.  I did not know this company had even assumed the responsibilities of collection of my outstanding balance, and had assumed Sprint had canceled, and would bill me as I had requested until Receivables Performance manage showed up as a past due balance of $440 on my Experian credit report.  I really do not understand why Sprint could not/refused to send me a final bill in the mail as requested.