I faced the same issue when I changed my mob to S23 FE. Two options Airtel customer care provided were,1) put another Airtel sim in the phone and check if the Smsc number get updated2) they will send configuration messages (CP messages) install them ...

Airtel right.. Go to any Airtel store.. They will give you a new sim. Call and other services will be activated in 4hrs in the new sim. Sms takes up to 24hrs.

Check if Smsc number is available. If not then check with service provider.. might need to update/replace the sim card