Call doesn't connect, no incoming, phone goes unreachable

My iPhone 4S was working fine and all of a sudden I noticed there were no incoming calls. I wasn't able to make any call. Friends started complaining that my phone is unreachable. Although I am able to send or receive text messages.

Got my phone reset completely but it didn't help. Went to the Apple Service Center and they reset again. No success. Every time it happens, I restart my phone and issue goes away for 15-20 minutes and after that its all the same problem.

I tried every possible way. Did a lot of research but nothing seems to work out.

Also, I changed the SIM to see if it works, there was no signal. It keeps SEARCHING.