Harassment and fraud is illegal, but people do illegal things all the time.

"court they can't get a judgment against you without you knowing about it"

... in theory, but in practice there are several ways to "serve notice" that don't involve the other party actually receiving the notice. Sure, if you can hire an attorney you may be able to get the default judgement overturned if you find out about it later, but again, it isn't all roses.

I'm not an expert on every states laws, but I don't think that the standard of proof for an uncontested claim isn't particularly high in most jurisdictions.

And to add to this, if a debtor doesn't get served (let's say that they took some bad advice from a friend and decided to dodge the constable trying to serve them in the hopes that "since I wasn't served, I can't be sued" is accurate), the constable will notify the attorney that contracted them to serve papers, saying that they couldn't serve them in person. At that point, the attorney sends a certified summons to the debtor's last known address and is then allowed to go to the court clerk and say that they've tried their best x number of times but the debtor can't be reached. They will enter their certified proof of mail into the record and the case will be tried on the scheduled date regardless, resulting in a default judgment when the debtor doesn't show up. Once that happens, if the debtor is employed (and even depending on the type of debt), they'll get a nasty surprise when their employer notifies them that their wages are being garnished and that there's no longer anything they can do.

I mean, I've been harassed by years for a debt that doesn't even belong to me, but rather to the person who previously had my phone number. Over the years I have learned their name, address, name of one of their family members, amount of the debt, and who the debt was originally owed to (a local government agency no less). The debt collectors shouldn't have disclosed any of this to me but they did, unprompted, often in voice mail. Mailed threats have been unsuccessful in stopping the calls, as when I send a nastygram to one collector they seem to just sell the debt on to another.

I guess my point is that the legal protections against harassment do exist but are not effective. It's not even my debt and I have a master's degree and generous income and I ended up changing my phone number in order to stop multiple calls per day. So to whoever ends up getting that number assigned, sorry about Esteban's $8k in back tuition and good luck.

The irony is that my new phone number also gets calls from debt collectors, but far less often. It seems the debt was either smaller or sold into a less predatory corner of the industry.