When you place a new or a existing SIM cardfrom another phone provider into a different cell phone and it asks for aSecurity Code then:-
This phone is locked to the original mobile phone provider. You need to contactthis mobile provider to ask for the unlock code (Security Code) for this phone.You need to provide proof of ownership etc and the phone's unique IMEI numberwhich is located inside the phone under the battery. They use this number togenerate the unlock code for your phone.
They should provide you with instructions on how to enter this code into thephone.
Once you unlock the phone, then you can insert a new SIM card in this phone orsign up with another phone provider using this phone.
You could ring the phone provider and ask for the unlock security code becauseyou are going overseas and wish to use your phone with a prepaid SIM card fromanother country. This may be a free service.
The other way to get your phone unlock is to use a phone unlocking company onthe Internet; they usually charge a couple of Dollars for their services.
You can also try this WEB site: - http://www.mobileunlockguide.com/mobile-phone/
For Nokia phone - try this WEB site too:-
Before start you should remove the SIM fromyour mobile, and also take the IMEI no . The IMEI number is unique the eachmobile phone.
The number is printed on the label under the battery.
You can also get the IMEI number by entering this on your phone
*#06# It will then display your IMEI number and you may also get a uniqueglobal serial number. Your IMEI number should be 15 digits.Hope it will help you.