Phreaking is the term that describes the activity of hacking into telephone networks for making free calls; it was prominent in the 1970s and involved manipulating phone systems with audio tones.
The term used to describe the act of hacking into telephone networks to make free calls from payphones in the 1970s is b) Phreaking.
Phreaking is a subset of hacking that specifically pertains to the exploration, experimenting with, and exploiting telephone systems. This often involved technically inclined individuals manipulating the telephone system to make free long-distance calls or to have calls charged to a different account. Phreakers would use various audio tones and signals to trick the system. The most famous technique they used was the blue box, which emitted certain tones to control the phone network. This practice became widely recognized in the hacker community and is considered one of the precursors to modern hacking. The term itself is a portmanteau of 'phone' and 'freaking' and reflects the technical curiosity and rebellious spirit of early hackers.
phone phreaking is a significant part of early hacker culture, where individuals exploited the telephone network to bypass the normal billing mechanisms.