Carrier Number Lock and Port Freeze to Prevent SIM Swap
SIM swap fraud relies on one weakness: a carrier will transfer your number to a new SIM card - or to a new carrier - if the request looks legitimate enough. A strong account PIN helps, but it is not a complete defense. Two carrier-level features exist to close that gap: Number Lock and port freeze. They work differently, and they matter.
Number Lock is a flag placed on your line inside the carrier’s own system. When enabled, it blocks any SIM change request at that carrier, even if the caller has your PIN, your billing details, or a fake ID. The carrier simply refuses to activate a new SIM on that number. The lock can usually only be removed in person at a store with government ID, or through a separate verification process. T-Mobile calls this "SIM Protection." AT&T calls it "Number Lock." Verizon calls it "Number Lock" as well. All three are free.
Port freeze is different. It blocks the port-out process - moving your number to a different carrier. A SIM swap inside the same carrier is not affected. Port freeze is a separate toggle, and it prevents a thief from taking your number to a competitor. Without it, a PIN alone can sometimes be bypassed by carrier employees overriding the system. Verizon offers "Port Freeze." AT&T offers "Number Transfer PIN" plus a port lock option. T-Mobile does not offer a dedicated port freeze; instead, it relies on its "Port Validation" feature, which requires a one-time code sent via SMS before a port can proceed. That code can be intercepted if a SIM swap has already occurred, which is a known gap.
These features are effective because they attack the two attack vectors. Number Lock stops the SIM swap. Port freeze stops the number from leaving. Together, they cover the full threat surface. A thief who cannot swap your SIM inside the carrier cannot intercept your SMS codes. A thief who cannot port your number out cannot steal it to a carrier with weaker security. The combination is stronger than a PIN alone.
All three major US carriers now offer some form of these features. T-Mobile's "SIM Protection" and "Port Validation" are available in account settings. AT&T's "Number Lock" and port lock are in the online profile under "Wireless" settings. Verizon's "Number Lock" and "Port Freeze" are in the "Account Security" section of My Verizon. Each must be manually enabled. They are not on by default. That is the catch.
A significant number of users do not know these features exist. Carriers do not advertise them prominently. You must log into your account, find the security settings, and toggle them on. The process takes about two minutes per line. If you have multiple lines on a family plan, each line must be locked individually. It is free, but it is not automatic.
There are limitations. Number Lock does not prevent a SIM swap if the thief has physical access to your phone and can use the carrier's app-based SIM swap flow. Some carriers allow SIM swaps through the app with only a password and biometric verification. If the thief has your phone and your password, the lock may not help. Port freeze does not prevent a SIM swap at the same carrier; it only blocks port-outs. And port freeze can be removed if the thief has enough personal information to call in and social-engineer the removal. The protection is only as strong as the carrier's removal process.
No carrier guarantees zero fraud. These features raise the bar. They make the attack harder and more time-consuming. That alone deters many thieves, who target easy victims.
The best practice is to enable Number Lock and port freeze on every line, then also use a strong carrier PIN or passcode. Do not rely on one layer. The layers are cheap and the effort is small.
These features are not available on every carrier. Smaller MVNOs often do not offer them. If your provider does not support Number Lock or port freeze, consider whether the risk of SIM swap is acceptable. The features exist because the risk is real.
Enable them today. The two minutes per line is the most effective single step you can take against SIM swap fraud.
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