a type of shortwave radio station characterized by their unusual broadcasts, which consist of spoken words, but mostly numbers, often created by artificially generated voices reading streams of numbers, words, letters, tunes or Morse code. They are transmitted in a wide variety of languages and the voices are usually female, although sometimes men's or children's voices are used.There's a lot to be intrigued by here. I mean, listen to this example; that is bound to bring up some questions. This NPR Story does a good job of introducing the concepts and answering some of the broader questions; I suggest you start there.
I suggest you end with The Conet Project - an open repository compilation of 30 years worth of collected Numbers station recordings.