Then tried removing the SIM card and turned on the device. It was working fine. Then set it to Aeroplane Mode and tried putting the SIM. There was no problem. Then when I disabled the Aeroplane Mode, the phone started to restart like a loop! So thought that it was an issue with OS(Gingerbird).
Then tried updating the OS to Sony's ICS update. After that, inserted SIM card. At first time when it is started, it was on Aeroplane Mode(which I enabled earlier). So, when I disabled it, the phone restarted. But after restart, it didn't get in the restart loop! But whenever a call is received, the phone just restarts !
Everything else is working fine! How can I fix this issue? Any help would be highly appreciated.