http://nemesis.lonestar.org/referenc...ling/dtmf.html DTMF (Dual Tone Multiple Frequency) http://nemesis.lonestar.org/referenc...ling/dtmf.html http://nemesis.lonestar.org/referenc...ling/dtmf.html nemesis.lonestar.org/reference/telecom/signaling/dtmf.html