Numbering resources are limited and DoT needs to recycle the unused ones for proper utilisation. When Airtel acquired Tata Docomo, they got their subscribers along with their existing number form that series, but Airtel did not get the entire number series (MSC codes) to assign to new subscribers, that went back in DoT's possession.
Now DoT has allotted it to BSNL to use for new connections in Delhi circle, as and when the Airtel (erstwhile Docomo) connections from that series get deactivated, they will come back to BSNL for assigning to new subscribers.
DoT has yesterday taken back the unused MSC codes of Aircel also in few circles to allot to other operators since Aircel has shut it's operations.