SIM swapping
SIM swapping is an attack in which someone fraudulently moves a victim's mobile service to a SIM they control, allowing them to receive calls and text messages sent to the victim's number.
The number moves, not the phone
The attacker does not need to steal the handset. They may impersonate the subscriber and persuade the carrier to activate a replacement SIM. The FBI also describes schemes involving carrier insiders and compromised carrier systems.
The useful result for the attacker is control of incoming calls and texts. If another service sends a password-reset code or an SMS sign-in code to that number, the attacker may receive it. Whether that gets them into an account depends on the service’s other requirements and the credentials they already have.
This is different from smishing, which uses deceptive text messages to influence the recipient. It is also different from making a false number appear on caller ID: a SIM swap changes who receives communications sent to the number.
Protect the carrier account and the accounts behind it
The FTC recommends adding a PIN or password to the cellular account and considering an authenticator app or security key for sensitive online accounts. Those alternatives do not rely on receiving an SMS code. Check recovery settings too: a stronger everyday sign-in method does not help a recovery flow that still accepts the hijacked number on its own.
Unexpected loss of service is a reason to contact the carrier through a known route, not proof of an attack. If an unauthorized change is confirmed, regain control of the number and review affected online accounts separately.
When the Phone Number Moves, Which Accounts Need Review? explains SIM replacement, port-out fraud, and the business accounts that may depend on a phone number.