Airtel — for getting new 4g sim of existing number

Address:

Mumbai City, Maharashtra, 400709

Dear sir/madam,
I am having the airtel prepaid connection from last 5 year, now i want 4g sim to use in 4g mobile handset, i have visited airtel store at vashi, new mumbai. They give me new blanck sim and told to send sms to 121 after completing all formalities they told sim will start with 2 hours but sim does not get started even completing 8 days. I again visit that store they told this sim is already used and now they don't have blanck sim so refused to raise my complaint. Please solve my complaint.
Thanks in advance.