SIM swap attack is a process through with a 3rd party gets an authorized access to your phone number, giving them access to your calls and messages through the SIM card.
SIM swap attack is the current reigning means of stealing from cryptocurrency investors' or traders' exchange account of those using their phone number as the authenticator to sign into their accouns. The scammers swap or steal your phone number from you to gain access to your cryptocurrency account and steal your fund.

How to protect yourself from such an attack includes
1. Keeping your funds in an uncompromised personal, custodial or hard wallet.
2. Using an app based authenticator like Auth, google authenticator, aegis and so on
3. Enable a SIM card pin protecting your SIM card from getting accessed without your consent.

Stay safe.