Voice modems extended dial-up hardware to handle audio calls, DTMF detection, and voicemail alongside data transmission.
Key Takeaways
Voice modems added a distinct command class (AT+V commands) on top of standard Hayes AT to control audio modes and speaker/mic routing.
Hardware supported modes including speakerphone, voicemail recording, distinctive ring detection, and simultaneous voice and data (SVD) on some chipsets.
Rockwell, Sierra Semiconductor, and PCTEL were dominant chipset vendors; driver and firmware behavior varied enough to make cross-platform support fragile.
Linux support through the mgetty+sendfax and vgetty stack made voice modems a cheap DIY answering machine and IVR platform in the late 1990s.
The same DTMF and audio pipeline that drove consumer voicemail prefigured later VoIP and softphone architectures.
