Received a phone call from this number.  The caller also left no voice message.  Who is this?

Called, heard a beep then nothing.

This number has called mulitple times and left no message. They're masking their identity on the caller id by just using Baltimore, Md. This is the kind of nonsense that was suppose to have stopped with the "Do NOT call" registry. What can be done to bring a class action lawsuit for this kind of illegal activity?