Follow-up - No message was left, so I again called back, let it ring once, then hung up. Sure enough about a minute later they called back and never left any message. Do they seriously think I'm going to talk to them without even knowing who they are? I'm just blocking them, sorry I couldn't find out more.

Called my cell phone (which is an Atlanta number as well) at about 10:30am on a Monday, but they left no voice message. I called it back to see if some recording would answer, but instead it rang normally. I hung up though after one ring, and they called me back about one minute later. Still waiting to see if they leave a message...