What are GSM Bands?
GSM technology can work on up to 4 frequencies: 850 Mhz, 900 Mhz, 1800 Mhz, and 1900 Mhz. Some GSM phones support all the bands or frequencies, or just a couple. A dual band phone supports only two, usually 900/1900 or 850/1900. A tri-band phone supports three bands, either 850/1800/1900 or 900/1800/1900. A quad-band phone supports all GSM bands. Quad-band phones are able to work with any and all GSM service providers. On the other hand, GSM service providers support just a subset of all the GSM bands available. Before buying a GSM phone, make sure the phone supports at least one of the bands that your GSM service provider supports.
