Debt collectors like to harass you.  They reach out to your friends, family, and do whatever they can to embarass you into paying regardless of whether the debt is valid or not.  However, there are limitations.  One limitation is that they cannot send written communications to you that have any words or symbols that indicate that they are debt collectors pursuant to 15 USC 1692(b)(5) of the Fair Debt Collection Practices Act:

A debt collector may not use any language or symbol on any envelope or in the contents of any communication effected by the mails or telegram that indicates that the debt collector is in the debt collection business or that the communication relates to the collection of a debt

If a debt collector violates this, they may be liable for statutory damages to you and also be required to pay for your attorney fees.