Injustice done by MTN and Mon

I am app**** for mobile phone contact in 2022, when the device was delivered at my home on Friday I noticed that it's not the correct device that I ordered. I immediately contacted MTN customer service and they introduced me to take it to any MTN store which I did the following day on Saturday. I was issued with a reference at MTN Carnival City as an acknowledgement of the returned device. After few months I was billed for the same device that I returned. Upon call and email to MTN about this matter both MTN and their subsidy Mondo are are playing marry go round with me and telling me to call Nudebt credit solutions and request them to remove my name on their bad credit record. I feel both MTN and Mondo are not doing justice to me hence their device was returned without being active for a second.