I am not an expert, but my thoughts would be to request exactly the name that sent the request for more info. If that person isnt available, leave your name and contact number and tell the person you are leaving the info with, that you are responding to their request that you contact them. If they ask what it involves, just state that it is private/protected info and would need to speak directly to them. With hippa laws, whoever you speak with, should understand and not hassle you to much, and if you mention you are returning their request to contact them, your message should get through.