First, your modem has to support Voice Command (V.253, IS-101)
Then, use AT+VTS to send voice tones (DTMF and/or dual tone with duration selection)
Or use AT+VTX to playback a voice (PCM,ADPCM,LINEAR,uLaw,ALaw...)
This way will be needed more commands (+VSM,+VLS,+VPR..., and <DLE> shielded codes).
Of course, do not forget the master command: AT+FCLASS=8 (select voice mode).
AT Command Set: to use the best search engine in the world - Google, you will find more and more.
If you are so lazy, try this one: **broken link removed**
To send tones via voice modem: try these by any RS-232 console:
Code:
DTE.Tx DCE.Tx
AT+FCLASS=8 OK
AT+VLS=4 OK
AT+VTS=0 OK
you will hear tones from the internal speaker of voice modem
To gather DTMF codes via voice modem:
Code:
DTE.Tx DCE.Tx
AT+FCLASS=8 OK
AT+VLS=0 OK
AT+VRA=0 OK
ATDT911 OK
now pick up the phone and press 123, the DTE.Rx will get: <DLE>1 <DLE>2 <DLE>3 ('<DLE>' as ASCII 0x10)