FNB

I would like to know about my case,fnb chose DMC as an external debt collector,DMC says i have settled my loans so i stopped paying this year January,but fnb agents insists that i still owe fnb,why cant fnb talk to DMC and get all my documents and do a thorough investigation of what transpired?Why should i be charged for something that i was told its settled and fnb has the decency to tell me that DMC should give me proof that they gave me a discount on my loan re-payment?