I know the default is to assume that iPhones warranties are only valid in the country where they're purchased, but there's an exception for India. It would have to be one that isn't service carrier specific (like my unlocked iPhone 7), which is pretty easy to find.
Apple has quietly introduced the international warranty facility for its phones in India, which means people who have bought iPhones in the US or any other country outside India will receive warranty privileges here as well. But, there’s a catch.
The warranty is only for iPhones that are factory unlocked.
In various markets, Apple sells phones in partnerships with network providers. The deals usually mean the phones are available for a low initial cost but buyers commit to stay with the provider for a fixed period. People often buy phones abroad using such plans and force an unlock, which enables the devices to be used in India. Such devices will not be eligible for the international warranty.