Mobile Fundamentals · SIMs & Identity · Lesson 4 of 4

Carrier Locks

Why some phones only work on one network.

Locked to a network

A carrier-locked phone only works with SIMs from one carrier. Put in a SIM from another network and the phone refuses to use it for calls, texts or mobile data, usually showing an error message. Wi-Fi, apps, the camera and everything else keep working normally.

Carriers lock phones because they often sell them at a discount or on a monthly payment plan. The lock stops a customer from taking a cheap or unpaid phone straight to a competitor.

Unlocking rules depend on the country. In the US, the major carriers follow an industry code. They unlock a postpaid phone once it's paid off, and a prepaid phone within a year of activation, and each carrier adds its own conditions. Canada went the other way in 2017 and required new phones to be sold unlocked. Other countries have their own rules, so check your local ones.

Locks matter for theft too. Carriers generally won't unlock a phone that's been reported lost or stolen, which makes stolen phones worth less. If you buy a used phone, check that it's unlocked and not reported stolen before you pay.

How carriers find your phone

You can change SIMs whenever you like, so a carrier can't use the SIM to recognise a particular handset. To unlock a phone, it uses the number built into the device, the IMEI.

When you request an unlock, the carrier asks for the IMEI. It checks whether it sold that phone, whether the phone is paid off, and whether it has been reported lost or stolen. If everything checks out, the carrier unlocks that specific device. Depending on the phone, this happens remotely or with an unlock code. Either way the unlock belongs to the phone, so it keeps working when you change SIMs or sell the phone.

The same number works in the other direction for theft. When a phone is reported stolen, carriers can add its IMEI to a blocklist. The GSMA runs a shared database so a phone blocked on one network can also be refused on other participating networks, even with a new SIM. Insurers and investigators check it too.

You saw this number in the first module by dialing *#06#. It's also in your phone's Settings and often on the original box.