becoz airtel is currently not providing wifi calling feature with this device. You can use jio sim ...it will work perfectly. becoz jio providing wifi calling features with most of the volte enabled devices but in case of airtel it is limited .
It not a matter of ota update from samsung in j7 nxt because this smartphone already has this feature.
use jio sim ..it works perfectly..becoz airtel has not started this service with most of volte enabled devices.