Phones have a screen-lock PIN/passcode. They only allow a certain number (typically 10) failed attempts to guess the code - and once the limit is reached they can "permanently" brick, or erase all data on device, or enforce some kind of (increasingly long and frustrating) delay before allowing any more guessing attempts. This reduces vulnerability to methodical brute-force attacks.
The PUK will lockout the SIM card (and the contacts, messages, apps, data, etc stored on it) before the device itself locks out, usually after 3-5 guesses. It's supposedly less inconvenient to obtain the PUK from the carrier than to replace a stubbornly locked phone.
But it's not at all convenient with Public Mobile because there's no store or kiosk, there's only the back-and-forth-questions-game-with-the-Mods thing which delays for at least a couple days (or even many days) before you can get the secret number and use your phone again.
The user PUK should be displayed in Self-Serve.
Or, at the very least, there should be some sort of "Request PUK" button which will email (to the Self-Serve registered email address) once a PUK has been assigned. Along with a"Remove PUK" or "Change PUK" button.
I think this might be the better way to go because some people might want PUK (in case, say, their kids/teens keep trying to swipe the phone and guess a path to the internet), while people flat-out don't want to allow reliance on carrier-side controls or useless PUK lockouts at all. (It can even be argued that PUKs reduce overall security because they won't allow the phone to continue to the point where it enables its own built-in security/encryption/locations technologies.)
