The call was automated and did not identify with any company. The message was to call 877-875-1912; the number to THE MORTGAGE SERVICE CENTER, Collections Department.
I got a voice message from these guys. They left a callback number -- 877-688-7114. I called (to ask to be put on their do not call list) and the automated menu wanted my SS# (which I gave as all 0's). I waited a few minutes to speak with an agent, who declined to tell me what the call was about until I identified myself, other than to say there was a problem with my Merrill Lynch line of credit. I asked to speak with the agent's supervisor, but was denied until I identified myself.
They called about my Charles Schwab mortgage, said they got a post office return and needed to verify my mailing address to get it out to me. I reluctantly provided my address, hopefully they were legit.