My number duplicated

This is the 2nd time I'm writing about this as MTN has not bothered to respond: have had my number for years, I don't even know for how long. For argument sake let's say that I have had my number for 15 years. I originally had a contract with cell c which changed to a prepaid after contract ended. I then a few years ago, about 6 or 7 years ago took an MTN contract and ported my cell c number.

Recently multiple people have been complaining that whenever they call me my number is being answered by someone else. I have also had situations where I do not receive OTPs etc and have often wondered why, and I tried my own from another phone number and someone else answered.

Mtn must answer this because I ported the number to MTN from Cell c. How is that my number is being used by someone else, and clearly that person can also access pertinent information through sms and if they understand what power they have, they can even access my emails etc by changing passwords if they can receive my sms'.

This is a violation of my personal information and must be investigated!!