i think its a problem with the modem chip. ive got a SG3 with similar problem. this phone arrivied this morning being water damaged.
i put it in the ultrasonic cleaner, brushed the board, few other tricks and got the phone working, almost perfectly... no imei number and says no sim card inserted.
been reading the forums all night, have tried injecting a new imei to no avail (cant get modem serial port to appear) and basically just bashed my own head in trying to figure it out.
so now im down to it being a shorted modem chip, and going to do triangle reset and see if i cant pass it in for warranty.. (6 months old)