Yeah I finally managed to unlock it. I entered the MCC/MNC number for the SIM I am using and it worked. But you need the unlock code to add them. I will post the steps later.
Here are the steps I followed:
Turn the phone Wi-Fi OFF and then turn device ON without SIM card.
You will see the NETWORK LOCK screen
In MCC/MNC field, enter MCC/MNC code for the provider you want to use. (example: enter 310260 if you want to use this phone for T-mobile USA Sim Card; 310410 or 310380 for AT&T). Check this link for complete MCCMNC for your provider: http://en.wikipedia.org/wiki/Mobile_Network_Code
(It is very important to try all the MCC/MNC code if your New Sim Card Network has more than one code)
In CONTROL KEY field, enter the Unlock Code
You should get a message: LOCK SUCCESSFUL
Turn off your phone, insert your New Network Sim Card and power ON. The phone should be unlocked.
Sent from my SAMSUNG-SGH-I777 using xda app-developers app