To activate the SMSC number you just need to go through the following steps:
Open your phone’s dialing app and dial *#*#4636#*#*.
A flash message will appear on your screen with multiple options.
There, click on the “Phone Information” and navigate for the SMSC option.
Now, click on the Refresh option and enter the new SMSC that you want to create.
Now, just click on the update button to successfully set the new SMSC number for your Airtel SIM.