Global System for Mobile Communications, or GSM for short (Turkish: Global System for Mobile Communication) is a mobile communication protocol. It is used by more than 2 billion people in more than 200 countries as the most common mobile phone standard. One of the most useful features is that users can make roaming calls from different countries with the same line. So in theory, if you don’t leave the coverage area, it is possible to travel around the world with your mobile phone without interrupting the phone call.