A DND registry lets subscribers block promotional SMS at the operator level; in India and Nigeria, among others, marketing to DND numbers is filtered by the operators regardless of the sender's consent records.
Several regulators run national do-not-disturb lists. A number on the list does not receive promotional traffic; transactional messages still go through. The filtering happens at the operator, so a sender's own consent does not override it, and the message is reported as blocked or, sometimes, silently dropped.
DND explains a chunk of the failure share on marketing campaigns to India and Nigeria, and it is one reason to keep transactional and promotional traffic clearly separated.
At SMSMeteor: DND-blocked messages are reported with the carrier reason where the operator returns one; transactional traffic is not affected.