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 …

Most cell phone service providers electronically lock the phone so that it can only be used with their service and there are reasons why they do that in particular, to force you to pay a high roaming charges while you are out of their network and using it in other part of the world.
This electronic lock is breakable and with some phones, it is possible to simply enter an unlocking password code to break the lock. However there are …