Does it ask for a 'PUK' code? If it does then you must get it from the Service Provider. Not the one thats written on bottom of the phone, the one who is actually your service.

try to put diff sim on your phone. if the phone is still asking for security code your phone is on phonelock. check this site www.cellularpager.com they fix, flash, unlock phone. email them first about your phone, and $100 is to much to pay just for code of this phone. they located in califonia.

It has a feature to lock the sim card, or the keypad, or the phone. My son just locked the phone - so i can't get past it asking for my code at power-on.
It is also At&T (written on the bottom of the phone) - if that helps.
We have SunCom service that was just switched over via a sim card change and the unlock procedure (which is different from the 'phonelock'). thanks!

is this a gsm phone? (at&t, cingular, t-mo)

My son locked his phone while he was away at camp, when he returned - he forgot what his code was. Now the phone is useless. I called Sony Ericsson and they said under warranty it will still cost $100 and must be sent to the factory. any suggestions? We've tried every number we can think of, and then some! thanks!