DTMF Generator
Forums
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1244333139
You Tube
Datasheets
http://wiki.sj.ifsc.edu.br/wiki/images/1/19/HM91710.pdf
http://www.sivann.gr/software/gps2dtmf/MT8880Trnscvr.pdf
http://www.holtek.com.tw/english/docum/comm/9200.htm