Like many other cell phones, Sony Ericsson phones allow you to program a four-to-eight number lock code into the phone, preventing anyone that does not know the lock code from making calls or accessing any of the phone options. You can enable and specify the lock code in the Sony Ericsson menu. Once it is set, you can reset the lock code to a new number by accessing the appropriate menu and entering the old and new lock codes.
Press the "Menu" button on your Sony Ericsson phone.
Select "Settings" and then choose "General" or "Advanced."
Choose "PIN codes" from the "General" menu, select "Phone protection" or choose "Security" from the "Advanced" menu, then select "Phone Lock."
Choose "Change code." Enter the old lock code, then press "OK." Note that the default Sony Ericsson lock code (if you have never changed it) is "0000."
Enter the new lock code and press "OK." Enter the new lock code a second time to verify it and press "OK."
Each Sony Ericsson phone is slightly different. Your menu options may be different from the ones listed here.
You can not retrieve the phone lock code, if you forget it, without sending your Sony Ericsson to a service centre. Make sure you to choose a lock number that you can easily remember.