The most common reason for this is that your recipients are registered with India’s NDNC list (National Do Not Call list).
Check if a number is registered in the DND registry
As per regulations, no telemarketing (Voice Call or SMS) is permitted between 9.00 PM to 9.00 AM local time (UTC Time: between 15:30 and 03:30). Also, this regulation affects the Sender IDs, which are replaced with an alpha code in the format “XY-NZZZZZ”.
Use this link to see if rejected numbers are in the DND list (remove country prefix when doing the lookups):
More information can be found here: http://www.nccptrai.gov.in/nccpregistry/FAQs.pdf