The document discusses the history and technology behind voice over internet protocol (VoIP). It describes how voice transmission evolved from analog to digital formats, allowing calls to be transmitted over data networks rather than traditional circuit-switched telephone networks. Key steps included sampling analog voice using pulse code modulation (PCM) at 64 kbps, compressing the digital signal, adding headers, and transmitting over IP networks using protocols like UDP and RTP. This allows voice calls to be made reliably over the internet instead of using circuit-switched telephone lines.