No service, changed IMEI, S/N 00000000

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http://forum.xda-developers.com – Hello, I have just bought an used Galaxy S3, but unfortunately I have some problems. The main problem is that I am unable to make calls. The phone shows No service and it doesn't matter whether SIM is inserted or not. Another thing I have noticed is changed IMEI. It is different than one behind the Battery. Also, serial number is 00000000 and it is very strange. I would be very grateful for any help! (HowTos)