For starters, I believe the Airtel Thanks app will always be running in the background and accessing our Wi-Fi or mobile data for this to work.
Also, if we don't have network coverage, we won't have access to mobile data either. And in most cases, no Wi-Fi either. It is very rare for someone to have no mobile network, but a working Wi-Fi.
Besides, we already do have the FREE missed call SMS alerts feature from airtel, which we can setup using the Call Forwarding feature in our mobile phones. We generally receive these SMS-es as soon as we are back into network coverage area.
Missed Calls Alerts is now Call Manager. App will notify you if someone tried to contact you when you were out of coverage. Additionally, it will also send alerts when someone you contacted is back in coverage.