GSM, which stands for Global System for Mobile communications, is the world’s most widely used cell phone technology. Cell phone carriers use GSM for their cell phone networks. Each mobile phone has a unique serial number, in case of GSM Phones, it is an “IMEI” number. The IMEI is only used for identifying the device, and has no relation at all to the subscriber. GSM also has the advantage of using SIM (Subscriber Identity Module) cards. The SIM card, which acts as your digital identity, is tied to your cell phone service carrier’s network rather than to the handset itself. This allows for easy exchange from one phone to another without a new cell phone service activation.
Therefore your SIM card will work with any compatible GSM phone, simply because your cell phone provider cares only about where to find your SIM card and not what phone model you use.
You can also use your GSM phone with any compatible SIM card, meaning you can use your GSM phone with any SIM Card from different networks around the world.
Although each network provider locks the phones they sell simply to make sure that you stay with them and use their service only, you can unlock your phone to be able to use it with any network around the world.
Get your BlackBerry unlock codes from N4BB here.