The US has a wider range of mobile technology standards, while India has only two: GSM and CDMA. GSM in turn comes in different frequency ‘bands’ which is why a tri-band mobile phone is the one you know will work with almost any GSM provider on the planet. Some service providers lock a cellphone instrument to only work with its own service as a marketing ploy or a sale pre-condition. Even if unlocked, some cellphones can be rendered useless if the unique serial number is blocked. Beyond voice calls and SMS, many features of a cellphone may depend on whether these are offered by your local service provider.