TTS is computer generated synthesized speech that is no longer the robotic voice traditionally associated with computers.
Real voices create the speech in fragments that are spliced together (concatenated) and smoothed before being played to the caller.
At that time, a system could store digitized speech on disk, play the appropriate spoken message, and process the human's DTMF response.
As call centers began to migrate to multimedia in the late 1990s, companies started to invest in computer telephony integration (CTI) with IVR systems.
IVR systems can respond with prerecorded or dynamically generated audio to further direct users on how to proceed.