Wakefield and Associates Consumer Law Violation? Aurora, Colorado

Got a phone call regarding a debt with a doctor's group in Aurora.  Did not believe I owed the money, called the doctor's group, found out they had sent it to collections, despite the fact I was paying them.  So, I paid it off with the doctor's group.  
Wakefield claims it's sent me written notice.  I never received it.  They confirmed my address.  Also, now they claim I owe something from a few years back.  Again, never got a notice.  Colorado consumer law indicates they're supposed to send out a written notice five days before they call.  Wakefield claims they've now sent out five notices.  I did not get any of them.

I suspect they are not sending out notices.  Just calling.  It is extremely irritating.  What consumer in his or her right mind would pay a collection agency, or anyone else who calls them demanding payment, when there is no written proof?