SIM Failure

I had bought Iphone 4S from the iphone store in India and it had been working absolutely fine for 16 months now. For the past 15 days I have been getting the SIM failure error everyday on my phone. For the 15 days, I used to switch off the phone for about 1 hour and switching it on again would solve the problem and I could use my phone normally and it worked fine. But the after sometime the SIM Failure error again used to come. Today the switching off and on is also not working.

I've tried using my phone's sim in some other Iphone and it is working fine. So it cannot be a SIM issue.

Please advice what can be done.

I'm still running IOS 5.1.1