What is GSM?

GSM means Global System for Mobile Communications. A GSM mobile phone is one that uses a SIM card and operates on the GSM network. If you have an unlocked phone or you want to buy an unlocked phone, it is or will be a GSM phone. Only GSM phones use SIM cards, and as locking solely prohibits the use of foreign SIM cards, only GSM phones can be unlocked. The frequencies that GSM phones operate on are: 900, 1800, 850 and 1900.