Based on the rule's wording as reported by India Today, carriers have six months to make sure the devices they offer are capable of installing the app. If any of the models in their roster still can't install it by then, they'll have to cut off its access to their networks:

"Every Access Provider shall ensure, within six months' time, that all smart phone devices registered on its network support the permissions required for the functioning of such Apps as prescribed in the regulations 6(2)(e) and regulations 23(2)(d). Provided that where such devices do not permit functioning of such Apps as prescribed in regulations 6(2)(e) and regulations 23(2)(d), Access Providers shall, on the order or direction of the Authority, derecognize such devices from their telecom networks."

Late last year, Apple agreed to help TRAI develop a version of the anti-spam app without some of its most worrisome features, such as its ability access to call logs. It's not entirely clear if the company can release the revised application within the next six months or if it has to think of another way altogether. As India Today notes, though, iOS 12 already has built-in anti-spam capabilities -- Cupertino might be able to use that to its advantage.