ATT collections

Over the last few days, I’ve recieved several phone calls from a weird ph # [no message].  I called the number back and it was Bay Credit (a collection agency).  Not sure what could’ve gone to collections, we did some research and found out is for our internet service that we cancelled.  We cxl’ed on Feb 18, and we recieved a bill statine payment is due on Mar 23.  I called ATT to find out how they could send to collections before the due date, and they stated the final due date is 9 March.  They sent to collections 15 March. 

Any suggestions for how I can get this corrected?  The person/ppl I’ve talked to are no help.  State I must call collection agency to make arrangements.