MtN - Debt I never took with them

I had a data only contract on a router, which ended & I didn't renew. The following months debit orders kept going off my account. I contacted them & they said I renewed & took a phone. I told them I never renewed & even when their agents called I told them I don't want to renew. I asked for details of this supposed transaction & git none. I escalated with their ***** unit & submitted all they requested. Never got further feedback from that department, instead debit orders also stopped. I assumed case was resolved. Now over a year later I'm being contacted by collections for an R42k for an mtn acc & apparently a blacklisting has been forwarded to credit bureau. I am livid to say the least this is *****ulent & unacceptable. They cannot ruin my financial record & have no answers for me.