Back around 2012ish, Digg.com had an article that was titled something like "Did we just find a secret MI5 number station?" I think this was a little after Black Ops 1 came out, so naturally I was super interested.
The article had a phone number. You call it, it played a short simple tune, a British guy said a string of numbers and nato phonetic letters, it played the tune again, and it was over. I called it again days later and it played the tune again but the guy said something like "please switch to channel alpha juliet echo romeo nine" then it played the tune again and was done. I think he might have even mentioned something about that channel being comprised, but I might be misremembering.
Super interesting all-in-all. Thanks, Digg.