Since there is no standard and there is no way to specify how human voice should behave in telephone calling and answering, it is difficult, if not impossible to automatically detect a voice call from a fax or data call. A common method to do this detection and switch is to have it answer with a human voice announcement first and then give the options to the calling party to push a number from the push-button pad to activate different features. In the meantime, if a faxtone (fax calling tone) is detected, the software and modem will then switch into fax mode. If no tone is detected then the modem will assume that this is a voice call and will continue playing the announcement message until it gives the caller the option to leave a message. After that it will wait for the caller to leave a message and if it detects that there is no energy level present during a certain amount of time it will timeout and the modem will decide the call is a data call and start modem handshaking. The shortcoming of this method is that some modems will get confused by the initial voice message announcement and will not get connected. Even if the data call is connected successfully, the longer handshake delay may not be acceptable in some applications. The modem can decide a fax or data call quicker if a data or fax calling tone is received. The voice announcement can be omitted in this case. ZyXEL has moved in this direction by having the capability of not only detecting and generating fax tones but also data calling tones.

Voice AT Commands.
ZyXEL Advanced Voice Capability.

Sam Leffler. Last updated $Date: 2003/11/03 22:01:51 $.