[Q] Cannot make calls

Hey guys, a couple of days ago I flashed CWM but did not succeed, had some signature verification issues. But now I have flashed back the stock rom from samsung and i have this very strange problem.

I am getting the network reception but i am not able to make calls. In fact, even the Internet is working (3G). I tried changing the sim card, but that doesn't work either. I am totally confused :confused: what could be the error.

Also when I hold the power button, my phone turns off

Please help me with these issues. Thanks in advance!!