Airtel — airtel 4g activation

Address:

Palakkad, Kerala, 678684

As a part o[censored]pgrading the Airtel 3G to 4G sim, I had purchased the new Airtel 4G sim from my nearby mobile store.
I had send the SMS to 121 as SIM xxxxxxxxxxxxxxxxxxxU (the number which was on my new sim).
I got the reply as they have received the request reply 1 to 121 with in 60 secs.
I had done it.
The again I got a message saying for confirming that u will get a call soon.
I got the call, I picked it as it said I have pressed 1 for confirm the sim change.
Soon after that I got a msg saying-
' Oops! Its an incorrect SIM number. Please SMS SIM to 121.'
I have purchased my new sim on 8/2/2019 and from that day I had tried this many times I didn't get it.the number which I have typed is same as the sim number in the sim.
I had approached the mobile store he told he is not having any idea regarding this issue.
I had called Airtel customer service 3 times but it was useless.

Please help me to correct it.