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Concept
Voice modem command set
Summary
A voice modem is an analog telephone data modem with a built-in capability of transmitting and receiving voice recordings over the phone line. Voice modems are used for telephony and answering machine applications. Similar to the Hayes command set used for data modems, in which the host PC commands the modem via a series of commands known as AT commands, there exists a well-defined set of common voice AT commands that are somewhat consistent throughout the industry.
Because voice mode is not the typical use for a modem, many modems on the market have poor or buggy support for their voice modes. Characteristics of a good voice modem depend greatly upon the intended application, and include:
Reliable operation. Many modems simply "lock up" or crash the host PC, though this is more common with Winmodems. Others have flow control bugs and other implementation bugs, possibly causing calls to hang, audio to skip, or audio to keep playing after an attempted abort.
Good audio characteristics. Some modems have an uncorrectably low signal volume or produce audio noise. Some modems are unable to recognize all but the best DTMF signals. Some modems do a poor job of recording, or detecting and reporting silence or the end-of-call voltage reversal, which some applications need.
Support for caller ID, if needed. "Type-1 caller ID" as used in North America is missing from the vast majority of modems. Nearly all modem chipsets support caller ID, but because the typical dial-up Internet user doesn't need caller ID, the extra components needed to support caller ID are often omitted for cost reasons.
Support for multiple instances. The drivers for many internal modems (typically Winmodems) cannot tolerate more than one of the same device inside a single computer. Symptoms of incompatibility include crashes, blue screens of death, or simple inoperability of all but a single modem. External RS-232-based (serial) modems do not have this limitation because each modem contains its own microprocessor and is unaware of other modems on the same host.
Official source
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