Sim swop while phone is on

Second time SIM swop is done on MTN number while phone is on. How is this possible? No one at the MTN shop can advise how that happened . Just told to go buy another SIM card to do SIM swap to get my number back. Number will be back after 24 hours and no other explanation.*****ent activity on my bank account.Please advise MTN how is this possible.I need answers