I need to build a dtmf control unit so when the secretary picks up the phone an press for example 00 and then 1 to 9 the the unit will close one of the relay 1..9 , # will close all relays.
There have to be thousands of these circuits availble. A bit of searching should get you partway where you want to go.
A few questions to ponder though: Is it a stand-allone system or an addition to the public telephone network? How do you recognize end-off-call (hang-up)? Station-to-station call capability? System-in-use indication? Single or multiple speech path? I could go on, but....E