Why was the whistle able to subvert the phone system in 1970?

In 1970, a simple whistle could subvert the phone system because of the technology used in analog phone networks, which relied on specific tones for signaling. Whistleblowers discovered that certain pitches and frequencies could mimic the tones used to indicate long-distance calls, allowing them to make free calls by tricking the system. This vulnerability highlighted the lack of security in the analog telephone infrastructure, leading to increased scrutiny and changes in telecommunications technology.