I called back and it looks like they had the wrong number on file. It's probably something to do with medical billing/health insurance if they don't bother to leave a message. In that case, you wouldn't want to them disclosing that type of info in your mailbox for security purposes anyhow.
I called and the husky sleepy voice asked, "Who are you?", I said, "I am not going to give you my name. Why did you call?". The man asked, "Are you evading someone? What is your name?" Then added, "Have a great day." They hung up.
I have someone else using my name and address but not my social (thankfully). I get a lot of collection calls for the other person. This caller was very polite and expedited the conversation by immediately offering the last 4 digits of the social for the person he was calling (not me). He apologized and said he would remove me from his calling list.