***** Sim swap with MTN

I bought a phone at Truworths early last year for my son and I was given MTN Sim card. Now around November some guy called claiming that the number my son is using belongs to him. We where so confused because when we bought the Sim it was ricad. Today the guy managed to Sim swap the number that belongs to my son, how did managed to do that nobody knows. How can MTN allows to Sim swap the number that belongs to someone and Sim that was Rica'd. Number that my son is using is ***.