A SIM swap is described in full under SIM swapping, which explains how the attacks are carried out, notable cases and how to defend against them. This page explains the shorter term, which is used both for the fraud and, in neutral contexts, for the normal process of moving a number to a new SIM card.
Mobile operators routinely transfer a customer's number to a new SIM when a phone is lost or upgraded. In a fraudulent SIM swap, a criminal persuades or bribes operator staff, or uses stolen personal details, to make that transfer to a SIM card they hold. Once the number is theirs, they receive the victim's calls and text messages, including one-time passcodes sent by banks, email providers and crypto exchanges. The tactic is a form of social engineering that often leads to account takeover.
Crypto holders have been frequent targets, because stolen coins are fast to move and hard to recover. Security advice is consistent: avoid text messages as a second factor where possible, use app-based two-factor authentication or hardware security keys, set a PIN or port-out lock with the mobile operator, and limit the personal details shared online. Some exchanges add a waiting period before withdrawals after a phone number changes. Other names for the same fraud include SIM hijacking and port-out scam.