I got a call from "Unknown" but had a number listed. When I call it back, it says the # is not in service

So, I get this call when I was in the shower and the number shows up on caller ID (from Mars Hill, ME), but name is "Unknown." I tried to call it back but the recording said the number was no longer in service. How can I find out who is calling me (this is the second call from this number). Oh, and when I did a reverse lookup, it says it's unlisted but it is a landline provided by Verizon. Curiosity is killing me.