That was not a very solid assumption on the part of the telephone company. Their only effort at security was to change the phone number on a periodic basis, at least once a year. Even so, the current number at any particular moment was very widely known among phone phreaks, who delighted in taking advantage of this convenient source of information, and in sharing the how-to-do-it with their fellow phreaks. The CNA Bureau trick was one of the first things I learned when I was introduced to the hobby of phone phreaking as a teenager.