A phone that supports four different radio frequency bands. Different mobile phone networks around the world use different bands. A phone can only be used on a network if it supports the correct band(s). Quad-band usually refers to a phone using GSM technology. A quad-band GSM phone is one that can be used for basic service almost anywhere in the world. That's because most countries have at least one GSM network, and most GSM networks worldwide use one of four bands. By supporting all four bands, a quad-band phone is compatible with most GSM networks worldwide.