DND means Do Not Disturb. This is an initiative by TRAI (Telecom Regulatory Authority of India) to save consumers from unsolicited calls/SMS. If a person registers his/her mobile number, then it would be illegal for telemarketers to send any sort of unsolicited SMS or calls on those numbers.
Hence, if you are trying to add a friend into @hikeapp and if his/her number is registered in DND, then s/he needs to opt-in to hike before you both can start communicating with each other. By opting in to hike, your friend is not opting out of DND, but merely allowing hike to send and receive messages from you.
How to opt-in to hike? A non-hike user can opt in to hike by giving a missed call to +919266630847 from his mobile. Dialing to this number, will automatically, opt in the DND user to hike and you can send him Hike2SMS messages.
Please do drop us a note in case you face any issues or need any further information, we would love to assist you.