Answer:
war dialing
Step-by-step explanation:
The technique that black-hat hackers use to do this is known as war dialing. This is basically when they dial a large number of telephone lines in order to gather data on the connected modems. This data is then analyzed in order to find weaknesses in the network and create a bypass in order to gain access to that network. Doing so grants them access to all of the personal data that the individuals connected to that network use on a daily basis. This is an illegal technique that is punishable with jail time.