GSM is a very widely used standard for communication over mobile phone networks.
Global System for Mobile Communications (GSM) is part of the second generation (2G) of mobile phone network technology. It was originally designed for two-way voice communication, but was later expanded to support more general data transfer, via GPRS and EDGE. It has been superseded by other technologies, but remains widely available.
Arduino support for GSM is often achieved by using the Arduino GSM Shield, although generic components can be used as well. In either case, a working SIM card and (typically) a contract or pre-payment plan with a mobile network operator is also required.
The GSM Library is available to facilitate programming.
Arduino projects involving GSM often focus on text-messaging. However, audio communication is also possible.