First is to do service!!!

logs:

then repair IMEI:
just reveal the real IMEI at the back of the Unit!!!
and Select SGH-X660 Cause they have the same PLATFORM...
But if you select SGH-U600 it will be failed(BUGS)....Instead if you load EEPROM....
Logs is here!

Unit is OK
tested many times:

if the problem persist!
Flash it....
Select default....
1.press write....(POP -UP message press power onn and *)
2.press only *
3.Insert cable!
Wait for secs....
4.Message at the NS pro....Power onn the unit!
5.remove cable,remove batt,insert batt and insert cable!!!
wait for minutes (All is well!)
Problem solve!