Airtel — Cannot turn on DND

I ported from Idea to Airtel. I get spam calls everyday. I want to turn on DND. But whenever I use the option from their website, they say this :
"An unexpected error has occurred. Our technical staff has been automatically notified and will be looking into this with utmost urgency."
I would have understood if this is a bug. It's been months after I received the message. Still, the same condition is there.
You are denying DND option. Fix this now.