Cellphone Unlocking: Can Your Phone Be Unlocked?
There used to be two cellphone technologies that were used by the majority of the world's mobile phone service providers: Global System for Mobile communication (GSM) and Code Division Multiple Access (CDMA). If you had a CDMA phone, then your phone was unlockable. However, if your phone operated on a GSM network, then it might be unlockable.
What's the difference? GSM phones used subscriber identity module (SIM) cards. A SIM card is a small card that's inserted into the phone. It contains all your contacts and settings, and it's linked to your account. You could take the SIM card out, put it into another phone, and if someone called your number, the new phone would ring. You could also put a different SIM card in your unlocked phone, and your phone would then work with whatever phone number and account was linked to that card. CDMA phones had no SIM cards and needed to be authenticated by the service provider, which made unlocking a phone impossible.
If a phone was locked, the service provider installed some software on the phone that tied the subscriber ID number on the SIM card to the serial number of that particular phone. If the SIM card and phone serial number didn't match, the phone simply wouldn't work. The SIM card wouldn't work in other phones, and the phone wouldn't work with other SIM cards.
If you lived in North America, you could usually determine whether your phone is GSM or CDMA based on the service provider you used. T-Mobile and AT&T used GSM, while Verizon and Sprint used CDMA. In Europe, almost all phones were GSM.
Currently CDMA is kept around mostly for legacy devices, while GSM has evolved into 4G LTE and 5G. Old 3G GSM networks are also being phased out in the coming years. Almost any phone now should be compatible with SIM cards.