I just called the number back and it's a collection agency. In my case, they had a wrong number and they told me they would remove my number from their list.
This number called my cell; I did not know the caller so I did not answer. While I was sitting here researching the number they called my work line. The caller Id identified them as Delta Outsource. I did not answer that call either.
I just got a call on my cell and when I didn't answer, the same number called my work. No messages were left. I called it back and it rang for awhile. Then a voice message came on saying no one can take my call and to leave a message.