According the the Fair Debt Collection Practices the collector cannot contact neighbors or family members if they know your name and phone number and could have contacted you directly. So if you have talked to them or they have your information than no - they cannot leave a message with a 3rd party, but if they cannot locate you the collector:
Cannot tell others you owe a debt or discuss details of the account.
Must identity himself, (by name, but not as a debt collector).
Must identity the name of the collection agency only if asked.
Can only contact the party once unless the collection agency has reason to believe the person has new information.
Cannot leave information about a debt on a third party's answering machine or voice mail service.
If they have violated this you can file a complaint with the Federal Trade Commission:
(877) FTC-HELP (877-382-4357)
Web: www.ftc.gov - Click "File a Complaint" on home page

