As I guess network issues are mostly from operator side. But you can check trying this once : Settings---connections---more networks---mobile network--network mode ---- GSM only (if 2g) & wcdma only (if 3g).
Else, you can do is check for the updates whether installed or not.
I also have faced this problem many a times. one of my friend suggested me to change my network. Now my problem has been rectified. suggest you the same.
