If you are a frequent traveler to international destinations say like to India then it is worth knowing some cell phone terminologies.
GSM or CDMA Most of the world follows GSM that is used by T-Mobile, AT&T and others as opposed to what US’s CDMA technology used by verizon and Sprint. It is always better to buy a GSM phone which is universal in nature.
Unlocked Phones Next stop is ‘locked’ or ‘unlocked’ GSM phones meaning that unlocked phones will allow any SIM (SIM is a small card used in GSM phone) from any service provider, anywhere in the world to be used in it. Most of phones in US from service providers like At&T, T-Mobile are locked so make sure your phone is unlocked for international use to put any SIM.
Band frequency Always go for quad band GSM unlocked phones only. Thetravelinsider.info have very good info on this topic – Two frequency bands are used by GSM services in the US. Two different frequency bands are used by GSM services elsewhere in the world. Us uses 850 MHZ and 1900 MHZ frequencies. Internationally 900 MHZ and 1800 MHZ are mostly used. quad-band phone is the phone that work with all four freqency bands mentioned above. In the travelinsider page there are phones with quad band and a table showing what countries use what bands.
Hope this basic info helps in choosing what unlocked phone to buy for international use.