The activation of DND on a phone number under TRAI guidelines is not binary, i.e. it is not just DND Active/Inactive. 

There are preferences that the end-user can set for receiving messages based on the category of the message, while DND is enabled for them.

If the end user chooses to receive SMS on select categories or for partially blocked registration, they can send SMS in this format:
 START < space> < preference no > to 1909.

There are 7 preference numbers to choose from:
START 0 - Fully blocked
START 1 - Banking, Insurance, Financial Products, and credit cards
START 2 - Real Estate
START 3 - Education
START 4 - Health
START 5 - Consumer goods and automobiles
START 6 - Communication, Broadcasting, Entertainment, and IT
START 7 - Tourism and Leisure

For instance, type START 4 to 1909, if you want to receive messages only on HEALTH.

Due to the above options, you may see inconsistent delivery for the same message sent to two DND numbers (this typically results in the status " PRFT-NT-MTCH"  populating for that number). 
Please advise the end user/customer to reset the preference as shown above and they will start receiving your Service-explicit messages.