Default on Credit Score for Non-Existing account

My husband (ID: ***) was busy checking his credit score and found that MTN has a default on his name for May 2020, for an unpaid account of R900. He does not have an MTN account or contract - it is ONE default in May 2020, no further "late payments" or updates. He has checked his records, and there are no emails or calls for this non-existent account, yet their default on his name is impacting his credit score. It's also impossible to speak to someone by calling them.