You have a very interesting case.
They just left a message stating that they're a law firm and gave a case number and a number to call back.
If they called you and was not able to talk to someone else, they have placed themselves in dilemma by leaving a message. Win win for you, lose lose for them. If they are indeed a debt collector, they need to include the disclosure that they are attempting to collect a debt in the ANSWERING MACHINE. This is because leaving a message in an answering machine constitute a form of "communication" :
Foti v. NCO Financial Systems, Inc., 424 F. Supp.2d 643 (S.D.N.Y. 2006), the debt collector left a pre-recorded phone message at the debtor’s home, saying “Good day, we are calling from NCO Financial Systems regarding a personal business matter that requires your immediate attention. Please call back 1-866-701-1275 once again please call back, toll-free, 1-866-701-1275, this is not a solicitation.” The message did not mention a debt, but only a “personal business matter.” The court nevertheless found this to be a communication within the meaning of the FDCPA, stating “The FDCPA should be interpreted to cover communications that convey, directly or indirectly, any information relating to a debt, and not just when the debt collector discloses specific information about the particular debt being collected.”
Hosseinzadeh v. M.R.S. Associates, Inc., 387 F. Supp.2d 1104 (C.D. Cal. 2006), the debt collector left several messages of which the following is typical: “Hello, this is Thomas Hunt calling. Please have an adult contact me regarding some rather important information. This is not a sales call, however, regulations prevent me from leaving more details. You will want to contact me at 1-877-647-5945 as soon as possible. This is a toll free number. Once again this is Thomas Hunt calling and my number is 1-877-647-5945. Thank you.” The court found the messages to be communications even though, again, they did not explicitly mention the debt. The court also concluded that the messages violated the requirement of § 806(6), 15 U.S.C. § 16926(6), that debt collectors provide meaningful disclosure of their identity.
--- Source: Consumer Law and Public Policy Blog
They also have to inform you of your other rights as indicated in FDCPA.
Additionally, since they are a "LAW FIRM" and mentioned the word "CASE NUMBER", a least sophisticated consumer (the principle used by most court) may think that they are being sued. If you are not being sued, I believe thats already a violation.
§ 807. False or misleading representations: A debt collector may not use any false, deceptive, or misleading representation or means in connection with the collection of any debt. Without limiting the general application of the foregoing, the following conduct is a violation of this section: ... The false representation of— (A) the character, amount, or legal status of any debt; or ...
