Do not respond to phone messages that press you to call a provided number, suggesting "legal complications" if not complying promptly. An example is a recent call made at Brown from someone claiming they represented a county process server's office, retained to deliver legal documents to appear in court. In this example, they said for some reason the papers were returned to the process server's office and were calling to advise that, to avoid any further legal complications or failure to appear in court, the person receiving the message must contact the attorney that is charge of the complaint at the phone number provided.
The AARP article How to Spot Debt Collection Scams provides a list of warnings to help you spot these as well as Do's and Don'ts should you receive one. See the Federal Trade Commission's article Debt Collectors Who Posed as Process Servers and Intimidated Consumers Settle with FTC for more background on this scam.