iPhone 6S A1688 supports all bands which will be used in india for 4G/LTE (FDD & TDD).... A1633 is for AT&T which needs an additional freq AT&T uses in the US that is not supported by A1633. its band 30 i.e. 2.3GHz used by AT&T in north america...boost mobile uses the A1688 model number of the iphone 6s. The only model that supports all LTE/4G (FDD & TDD) frequencies in India is the A1633. Many cases of other models not supporting Airtel 4G in India; look up apple forums. So do your research before you buy