Unauthorised Invoicing & Blacklisting Threats

I received a phone call last year from someone claiming to be from Telkom, stating they needed to update my contact details. I provided my updated information, assuming it was a routine update. However, this year, I started receiving final demand letters from Trudon, and upon investigating, I discovered they have been charging me R460 per month for a service I never signed up for. At no point did I agree to any service or contract with them. Now, they are threatening me with blacklisting over an account I never authorized. This is unacceptable, and I demand immediate resolution.