What are your rights against bill collector callers who do not identify themselves hang up and threaten to sue?

Collection agencies are governed by federal and state laws. Debtors are given protection under the Federal FDCPA. A collection agency cannot "threaten" you with anything. They can inform you that your account may be referred to a collection attorney and legal action may be initated. Please familarize yourself with the laws pertaining to collection agencies. Be advised, that some of the laws do not apply to the original creditor.