Samsung S3 Network lost randomly

I just bought Samsung Galaxy S3 At&t version. Phone works fine but I have a strange problem. When someone dial my number the they receive message this number is unreachable. At the same time if I try to make a call from my s3 it is also not connecting. but if I restart my phone I can receive and make calls and than again after some time it start doing the same problem.

Messages are also not comming immediately instead 10 to 15 message are comming simultaneously.

Phone show full signal bar, and I have never face this problem on other phones

Sim is sitted properly in mobile

Kindly someone tell me if this is the phone problem or something else

Thanks