War dialing refers to the use of various kinds of technology to automatically dial many phone numbers, usually in order to find weak spots in an IT security architecture. Hackers often use war dialing software, sometimes called "war dialers" or "demon dialers," to look for unprotected modems. Narrowing down the list of phone numbers answered by modems cuts down the amount of time it takes to do this.
Techopedia explains Wardialing
In some senses, war dialing is like the predictive dialing used in call centers. However, where the aim in call centers is to find human respondents for actual telephone conversations, war dialers are generally used to find electronic lines of communication, either to penetrate systems, as mentioned above, or to aid in network security by identifying unprotected modems and adding security.
New types of war dialing software may allows users to get more information on the many phone numbers on a call list. Speech recognition software can help these automatic dialers to get more data about all of the numbers in a given set, and this valuable data can be important to users. However, some U.S. legislation has curtailed the practice of war dialing by making certain kinds of "dial for tone" programs illegal.