hacked number

Good day

I would like to find out if when a customer does sim swap what security question that the customer is being asked yesterday I had network problems, and I called mtn to check if what is happening and all to find out someone did sim swap with my number, and the person hacked my whatsapp and when I was busy with my phone it popped that I need to verify my contact number as if its updating whatsapp, as I was busy with that its says it will send the OTP number and its when I noticed the network its not there and the person that hacked my number send some whatsapp to people that I need R1500.00

my question is this how is the sim swap being done how the contacts no still exists if I did sim swap with new sim card, because last time I did that I lost all the contact it was many years back. what are the verification details are being asked in order to know that is the right person of the number.

and when I called mtn to ask to block that person and recover my number back to me, they asked me to get the numbers that are in the back of sim card off which I couldn't provide because they are rubbed off, and they told me that they cant help me and I must buy a new sim card, amazingly I went to mtn store and they managed to get the number of that sim on their system, so how can the people that were helping me via call center couldn't check that number and be able to assist me,

***** done on my name, because someone did transfer money to this person that hacked my number.

I want to know what security we have with our sim cards, I was told they cant block the number I don't own I just use the number and the person even managed to do advance of R45.00 and I had to pay that airtime