Security master code not working - Nokia 7510 Cell Phone

Changing the security code
Of course, if any potential phone thieves are reading this, they'll now know the default security code is 12345.

To change the security code, go to Menu > Settings > Security > Access codes > Change security code. When prompted, enter your current security code and click OK. The enter your new security code (a PIN between 5 and 10 digits in length), click OK and enter the new code again to verify it. Click OK and you'll see the message 'Security code changed

or
*#7328748263373738# resets security code.
Default security code is 12345