GSM is the most popular standard for mobile phones in the world. GSM phones require a small electronic chip, called a SIM card, to be inserted into a slot in the handset. This international SIM card is the heart of your GSM phone - it contains your cell phone number, your phone book, and all of your registration information. Most companies with GSM service provide you access to over 320 GSM networks in over 140 countries.
You may heard about Unlocked GSM phone. An unlocked GSM phone is either 1) a phone which is directly from the manufacturer which was never locked, or b) a phone from a carrier which have been modified to openly work on any network. Having unlocked phone, you don't need to be stuck with certain network.
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