I had a Cisco SPA525G handset that when I went to reset it, it asked for the password, which I did not know.
Thankfully there is a workaround for this.
Reboot the handset, and after the Cisco logo has gone and then the animation shows, press the speaker button 3 times.
Press the speaker button 3 times when you see this:
You will then see this hidden menu option:
Select Recovery Reset and you’re done.