Sim Swap *****

I have experience a worse service ever from MTN yesterday 30/03/2018. I received a call from a person who pretended that he is from MTN, he mentioned that someone was trying to do a sim swap on my cellphone and he asked me to verify few information and he gave me a code to punch on my phone of which I didn't because I suspected that he might be a *********. I drop the phone an I phoned MTN around 13:08 using number ********** 026 to verify if they did phone me. They said MTN will not phone me no matter what and there nothing they can do as this is a normal issue. Its ok because I didn't punch the code they gave but to my supprise around 18:00 my phone number was not working and I did phone MTN and they did confirm that sim swap was done around 15:54 without my concern. Why they did not put a stopper on my cellphone number because I did report before they can do a sim swap. Now as a result I lost R2700 from my Nedbank account to these **********. I NEED MTN TO TAKE A SERIOUS INVESTIGATION REGARDING THIS CASE AND I NEED TO BE COMPENSATED BECAUSE ITS CLEAR THAT THEY DONT PROTECT OUR CELLPHONE NUMBERS. Anyone can just do a sim swap and MTN allow that transaction to go through.