Although we aren't really sure when exactly the first 711 number showed up, they've been kind of a phreaking anomoly over the years. They were spotted in the letters section of the 1997 Spring issue of 2600 Magazine, and have appeared on many popular phreaking forums since then, like BinRev in 2005 (https://www.binrev.com/forums/index.php/topic/11638-number-i-found-scanning)