I haven't owned a nokia phone for quite a while, but this method definitely worked on both monochrome nokia phones I owned. These are the variety with snake built-in.

When you're at the lock screen, just type in *3001#12345# and now you're at the service menu. All you have to do is scroll down to the menu item for the lock code, select it, and your super secure lock code is now staring back at you in plaintext. Power cycle the phone, type in the code, and you've now got an unlocked phone.