I stay in India and have just bought the unlocked version of S3 (SCH-i535).
All is well with the phone but Mobile data is not working for me.
When ever i try to switch the Mobile data on i get the following messgae "Insert SIM card to access network services"
I have not rooted the phone yet and its on pure stock 4.1.2.
Sent from my GT-I9300 using xda app-developers app