Strange problem with Samsung ToccoLite GT-S5230 - Please HELP

euglycaemia

I have a Samsung ToccoLite bought 2 years ago which was unlocked when I bought it from DialAphoneI gave this to my brother who is India and was using on a local network 'AIRTEL' until now with no problem. When he bought a new simcard on the same network and put it in the phone, he is getting ''NO SIM MODE'' message. I tried googling and it seem to be a common problem and they recognise this is locked phone.Strange thing is , if it was working on a AIRTEL for more than a year, how can this get locked for the same network.Any idea how to unlock this?Please help, as my bro is left with no phone now...