GSM-900 and GSM-1800 are used in most parts of the world (ITU-Regions 1 and 3): Africa, Europe, Middle East, Asia (apart from Japan and South Korea where GSM has never been introduced) and Oceania.
Further GSM-850 is also sometimes called GSM-800 because this frequency range was known as the "800 MHz band" (for simplification) when it was first allocated for AMPS in the United States in 1983.
In North America GSM-1900 is also referred to as Personal Communications Service (PCS) like any other cellular system operating on the "1900 MHz band".
The result therefore is a mixture of usage in the Americas that requires travelers to confirm that the devices they have are compatible with the bands of the network at their destination.
A new addition has been the quad-band phone, also known as a World Phone,[5] supporting at least all four major GSM bands, allowing for global use (excluding non-GSM countries such as Japan, South Korea and as well countries where 2G system was shut down to release frequencies and spectrum for LTE networks like Australia (since 2017), Singapore and Taiwan (since 2018).
For example, one version of the Nokia 6340i GAIT phone sold in North America can operate on GSM-1900, GSM-850 and legacy TDMA-1900, TDMA-800, and AMPS-800, making it both multi-mode and multi-band.
This allows the same handset to be sold for AT&T Mobility, Verizon, and Sprint in the U.S. as well as a broad range of GSM carriers worldwide such as Vodafone, Orange and T-Mobile (Excluding-US), many of whom offer official unlocking.