Phone does "Not Register To Network"?

I am using Samsung note 2 and from Dec. 14 I am facing a serious problem of signal drop. Some times the mobile work fines. But some times signal get drop to zero. My friends do complain me that my mobile is switch off or out of range whenever they try to contact me. Whenever i try to call i get error "Not Register To Network. Even if i try manually connection to service provider network it doesn't get connect. Even i change my sim card but still problem persist. I am facing this problem after upgrade I received in Dec 14. Even I re-installed the firmware at Samsung service center but still problem persist. The person at samsung service center suggested me to change motherboard and it cost me almost a new phone. Can any one suggest what the issue. Is it software or hardware issue. Waiting for your quick response and suggestion.

Thanks & Regards,

Rakesh.