My Samsung Focus and my sister's Omnia M both have this problem. And it hasnt got anything to do with Airtel (or Nokia) as I faced the same issue with Docomo. The signal drops and doesnt come back until you turn the Flight mode ON and then OFF or restart the phone completely. There's something wrong with the OS and not the phones as far as I can gather.
Facing the same issue with Vodafone - their signal takes its sweet time to come on. Wondering what to do. Have not faced this issue with Docomo. Samsung Focus - ATT 7.5