Because Sony Ericsson no longer exists (merged into Sony Mobile, which is now largely defunct), there is no active legal enforcement against using this tool on vintage hardware.

Dial *#06# on your phone to find the 15-digit identifier.

Even with a correct download, you may encounter issues:

However, the version number "v2.3" is significant. In the cat-and-mouse game between phone manufacturers and unlockers, algorithms change frequently. Sony Ericsson frequently updated their security to block these calculators.