Blacklisted for an account that I was not made aware of

I was admitted last year February at the dermatology unit for 6 weeks and there after I go every 3 months for my treatment. When I was admitted I updated my address, cellphone number and employment details as they had changed. I checked now on my credit profile Groote Schuur has put 2 defaults on my profile for non payment. I never got a single call, account from the post or anything pertaining to what I owe and I don't understand because you can't blacklist me if you didn't attempt to collect your money from me. I go there every 3 months and non of the admin staff ever mentioned anything about a bill owing. I want to apply for a bond and I was declined because of the 2 defaults that were added on my credit profile