Please note that this method of unlocking without a code is not very reliable.

Insert SIM from the network that it is locked to.
Turn on phone.
On home Screen, slide up phone.
Type 2945#*#
ENG. Mode menu should appear.
Go Down to Factory Reset. Select it. May take a few minutes to reset!
This will reset the phone back to before the phone was locked to a network.
The phone will automatically restart (the network logo at start up …