A SIM swap happens when an attacker convinces your mobile carrier to transfer your phone number to a SIM card they control, intercepting your calls and texts.
Many accounts use SMS-based two-factor authentication. Once an attacker controls your number, they can receive your reset codes and take over your accounts.
With access to your email and exchange accounts, an attacker can reset passwords and withdraw funds before you even notice.
Use an authenticator app instead of SMS for two-factor authentication, set a PIN on your carrier account, and avoid sharing personal details that help attackers impersonate you.
Never rely on SMS for the security of financial accounts. A hardware key or authenticator app is far stronger.