MTN Weak security

On 02 November 2023 I received a call from a MTN number and the person said he is from MTN and asked me if I am HB Sibiya and I said yes but I cannot talk now I am in a meeting, after that my network was gone and I thought it usually happens and I switch off and on by phone but I didn't help and people said they were looking for me but they can't find me on personal number. I saw that something was wrong in the evening when I put my Sim card on another phone and it didn't work still. I went to MTN at Vaal mall the following day and the lady did confirm that someone did a Sim swap on my number, I showed her the number and explained what happened and she said there is nothing they can do. She reactivated my number and she said I must call 135 to do security questions to prevent any unauthorized swim swap and I didn't know about this. Just by saying Yes to a call