MTN ******** me

I have phone contract with mtn from 2020 that is going to end this year August, so last month I started receiving massages from MTN saying I owe the R2900 that I dnt know where it was coming from because I was paying R349 every month and I was paying them very well.. So I went to their store to find out about R2900 and the lady there said she can't see where it's coming from but it is appearing on the system that I owe it, the lady did not help me she just advised that I call the call center maybe they will help. I did call mtn call center and the lady said she will put it on investigation they will send me a message so that I can call them again when they are done investigating, so I waited for about 5 days then call again and still not done, but my biggest problem is they are still sending me massages saying I must avoid legal actions by paying the money, now it's been 2 weeks waiting for answers..