I bought a S3 and it came locked to an unknown carrier. All the unlock services require the information of which carrier the phone is locked to give me an unlock code. So, how can I find that out?
Depending on where you are in the world, and which carrier you're with, your phone will have a unique model. You can find your phone's model number by going to Settings > About device and it'll be around the middle of the page.
North American models
Carrier Name Model Identifier AT&T (US) SGH-I747 Bell Mobility (CAN) SGH-I747 Moblicity (CAN) SGH-T999 Rogers Wireless (CAN) SGH-I747 SaskTel (CAN) SGH-I747 Sprint (US) SPH-L710 T-Mobile (US) SGH-T999 Telus Mobility (CAN) SGH-I747 U.S. Cellular (US) SCH-R530 Verizon (US) SGH-I535 Videotron (CAN) SGH-T999 Wind Mobile (CAN) SGH-T999
Via techtrickz.com: you can type
*#7465625# on the phone to obtain the lock status.
Type the code *#7465625# or *#SIMLOCK# on the keyboard of your phone. It’ll display various lock status information of your phone. This method should work on all other Samsung Android handsets.
They also suggest imei.info but the site seems to have limited functionality recently.