Given that the US Supreme Court made the obscene mistake of saying that 3rd party debt collectors don't fall under the Fair Debt Collection Practices Act, expect harassment like this to become more common.
They were simply confirming the plain meaning of the FDCPA, which applies to people who collect debt that is owed to another person. Junk debt buyers buy their debt outright, so they're not collecting on someone else's behalf, so FDCPA doesn't apply. If people are upset about this, they should direct that toward Congress for not having amended the law.