In simple words, TRAI wanted to ban use of devices that didn't allow installation and use of its DND app. The app description reads, "TRAI DND app will help consumers to curb unwanted Telemarketing Calls/SMS by reporting to their respective telecom service providers".
Apple's stance against the app was due to privacy issues, as Apple wanted to make sure that the app, despite being government-regulated, did not provide a user's call or message logs to the authorities.
After a prolonging rift between the Telecom Regulatory Authority of India (TRAI) and iPhone-maker, Apple, the do-not-disturb (DND) mobile application of the former has been available on iOS since Friday. This will allow the app to receive information on your calls. According to laid out guidelines, any updates have to go through an iOS App Store so Apple can verify the integrity and safety of the new versions being rolled out to the public.
Because of privacy concern, Apple has denied releasing the app on App Store.
Unwanted calls were tackled with TRAI's initial release of DND which Google allowed in its Google Play Store back in 2016. To avoid this, users with the app can report those calls and messages which will then be blacklisted by TRAI by putting them into a "do not disturb" registry.
"Trai DND App will not block any SMS/Calls from authentic businesses/entities that you have subscribed too, e.g. your bank, food delivery apps, travel/taxi apps etc".
This can be done with spam SMS as well, through a "report message" option just below an SMS from any sender that is not in your contact list. And now, the app is available for iPhone and iPad users as well. In order to install the app, you must have the latest version of iOS 12.1 installed. Once a user selects an unwanted call/message, DND will auto-generate the report and send them to the relevant carrier.