I recently discovered a feature that's part of the command set of some voice modems that enables it to produce tones. The latter is of interest to us since the US dial-tone and others are a combination of two frequencies (440Hz and 350Hz in the US).
Below is an ordered list of AT commands that initialize the modem, produce a dialtone for 2.5 seconds and answers the call. To check if the "AT+VTS" command is supported, or to get the parameter value ranges supported by it, use "AT+VTS=?"
# resets the modem
ATZE1
# puts the modem into voice mode
AT+FCLASS=8
# opens the connection
AT+VLS=1
# produces the tone
AT+VTS=[440,350,255]
# closes the connection so it can answer
ATH
# answers the connection
ATA
* Usage: The first two parameters are the desired frequencies. The third parameter is the duration of the tone and is the desired time (in milliseconds) / 10. If the square brackets do not work, try using "{ }" backets as an alternative.
* Note: Some voice modems use "#" instead of the "+" character for the VLS and VTS commands. If the modem is returning "ERROR" after sending either command, try replacing the character.
from http://dreamcast.onlineconsoles.com/php ... 9950#79950