DLT (Distributed Ledger Technology) registration is India's mandatory system under which every sender, header (sender ID) and message template must be registered with a telecom operator before commercial SMS is delivered.
Since 2020 the Indian regulator TRAI requires senders of commercial messages to register their entity, their headers and each message template on a DLT platform run by the operators. Unregistered headers or templates that do not match are blocked. Transactional and service messages have their own categories with fewer restrictions than promotional ones, and the national do-not-disturb registry applies to promotional traffic.
For a foreign sender, registration usually goes through a partner and takes time; until then, messages go out from shared senders where the route allows it.
At SMSMeteor: India is marked "registration required"; the India guide explains what delivers before and after registration.