Product: Debt collection

On XX/XX/2018, I found out on my credit report an incorrect account from PlusFour Inc. for past due amount of {$2800.00} as of XX/XX/2018. 
On XX/XX/2018, I disputed the collection account with XXXX  and XXXX. As of today, I received the report of the investigation done by XXXX  stating that the adverse account will remain on my credit report because PlusFour certified that information reported were accurate. All without providing me any written evidence by which PlusFour could support its false statement. 
After receiving the 'Collection Notice ' from PlusFour on XX/XX/2018 ( one and only written correspondence from PlusFour ), I disputed a validity of a debt by sending a certified mail letter on XX/XX/2018 to the address shown on the Collection Notice ( XXXX XXXX XXXX XXXX , XXXX XXXX, XXXX XXXX , NV XXXX ). The same dispute letter was sent to XXXX XXXX XXXX  and to XXXX XXXX XXXX and XXXX. While doctors ' office and XXXX accepted the letter sent to their notice, PlusFour REFUSED TO ACCEPT my letter disputing a validity of a debt! 
On XX/XX/2018, I resent the dispute of validity of a debt by sending the second certified mail letter to the address available on the PlusFour website ( XXXX XXXX XXXX, XXXX XXXX, NV XXXX-XXXX ), and the dispute letter was returned to me because an attempted delivery failed due to the addressee NOT KNOWN. PlusFour 's  website provides only the mailing address ; there is no online form that can be filled by people like me. 
If PlusFour accepted my letter disputing the validity of a debt, they would know that the charges for specialist in-patients visits that I NEVER REQUESTED were still under the review and an appeal of my insurance. 
I received the FIRST BILL from XXXX XXXX XXXX  at the end of XX/XX/2018! 
Based on my knowledge, the majority of the charges for 17 specialist in-patients visits were denied by my insurance because the doctor FAILED to obtain ( not once, but 17 times!!! ) a pre-certification for his repeated UNILATERAL non-emergency visits. Based on my records, my insurance after all re-processed all claims after my second appeal ( considering my serious health situation and hardship ) for 17 NON-EMERGENCY specialist in-patient visits that I NEVER requested!!! 
On XX/XX/2018, my wife called the billing department for XXXX  XXXX XXXX  to learn why I received the bill from their doctor, and she explained the whole situation with the review and a filed appeal with my insurance and my intention to wait for the result of the appeal for the charges for 17 specialist in-patient visits that I never requested. They were aware the appeal could take 45-60 days to be processed if not longer. 
Based on my records, my insurance processed the claims and paid to XXXX  XXXX XXXX {$680.00} by sending checks in XX/XX/2018. 
Around the middle of XX/XX/2018, XXXX  XXXX XXXX  sent me a new bill for {$550.00} ( with costs of services {$2700.00}, showing an adjusted amount of {$1500.00}, and being paid {$680.00} by my insurance ). The bill generated on XX/XX/2018 is a proof that I did not owe {$2700.00} to XXXX  XXXX XXXX as PlusFour continues falsely reporting to credit bureau agencies and even falsely reported to XXXX  the accuracy of that information on XX/XX/2018, in a response to my dispute with XXXX! 
On XX/XX/2018, I replied to the bill ( through certified mail ) with a letter requesting from XXXX  XXXX XXXX  evidence of 17 specialist in-patient visits and medical notes/reports/test results, etc. that were allegedly provided every 3-4 days, and for which I am allegedly responsible to pay copayments ( including to provide me the name of the XXXX  employee that arranged for those specialist in-patient visits, without telling me or asking for my approval ). I didn't see any doctor visiting me 17 times during my stay at XXXX. 
As a patient, I am ENTITLED to an explanation and understanding for what and why I was billed. In my opinion, doctors should not be allowed to put any bill that is in a dispute for collection, and if they do, the collection agency HAS TO ACCEPT A WRITTEN DISPUTE of a validity of a debt before reporting any debt to the credit agency and verify the validity of the debt. 
As of today, I still didn't receive any response from XXXX XXXX XXXX. It seems that I, as the patient, have no right to know why the doctor that I never chose ALEGGEDLY appeared for in-patient visits every 3-4 days and what he ALLEGEDLY did every 3-4 days. I AM STILL AWAITING THE EXPLANATION AND THE ANSWERS to my questions asked on XX/XX/2018. 
Based on the phone call with the representative of XXXX on XX/XX/2018, it's allegedly the PlusFour 's policy to refuse the certified mail correspondence received from people. Under Nevada law, I have a right to dispute the validity of a debt in WRITING within 30 days of receiving a collection notice. I made a WRITTEN DISPUTE OF A VALIDITY OF A DEBT, but PlusFour ignored it by its refusal to accept certified mail. In my opinion, by implementing its non-acceptance of certified mail policy, PlusFour doesn't want people to dispute a validity of a debt, and PlusFour doesnt want to go through a hassle of obtaining a verification of a debt for a consumer. 
I never in my life dealt either with a doctor that IGNORES the requirement of the pre-certification of non-emergency visits or with a collection agency ( or anyone ) that REFUSES to accept a certified mail letter. 
In my current experience, PlusFour HIDES from people behind its non-acceptance of certified mail policy, and XXXX  XXXX XXXX  ( doctors ) CONCEAL from PlusFour receiving the payment of {$680.00} from my insurance, as well as the fact there was the {$1500.00} adjustment to charges, and doctor 's irresponsiveness to my request for the explanation of his 17 non-emergency visits and to my request for the copy of medical notes/records/results, etc.. And I, as the consumer, am left to deal with the credit bureau agency that doesn't care about the proper investigation of the dispute as well.