mobile

hi ,
im new here and that's my first time .
I have galaxy s3 i9300

recently im facing a big problem that accuring suddenly -
suddenly the phone isn't getting phone calls and I cant make a phone calls .
in other words , someone who try to call me , is getting the "responding massage" .

when im looking inside the phone at the settings , I see that the network isn't connected , and therefore I need to search for my network bymyself . sometimes it works , and sometimes not .

I tried to change the sim card to a new one and even to another sim card and still the problem is continues.
I tried to Restore factory settings , but no use .

I have to say that at the time that the phone is losing connection with the world , I can see on the right top of the phone that I have "Absorption" .
also at that time im getting Wi-Fi .

I have 4.3 android
the imei is normally - 353976.....

I don't no what to do , please advice me