Vodacom phone falsely blacklisted

I'm at impasse regarding my mobile device: during level 4 my MTN sim stopped working. MTN referred me to Vodacom, saying my phone was likely incorrectly blacklisted. It is a Vodacom branded galaxy s7 phone. Vodacom sent me many messages, all saying that the issue is resolved and call closed... but it's still not working. The last time I was at 4u store, they said it's due to bad debt or unpaid bills... But I've not been a Vodacom customer for more than 3 years and there are certainly no unsettled bills. Whatever contract the phone was purchased on originally has long since expired. It seems my accounts are mixed up with someone else's, but none seem able to correct this issue. In the meantime I'm unable to use the services I pay MTN for. I don't want to buy a new phone to resolve this problem!