When conducting testing of the project to reset a Cordless Phone Off hook with no audio, the annoying noise present when a phone is left off hook usually preceeded by "If you would like to make a call, please hang up and try again", that noise was not detected by the Audio Detector, but that is OK as normal audio keeps the Cordless Phone Base from being reset.
In choosing the I/O pins for the type and function, I know that any pin can be configured as a digital input or output, but only designated pins can be an Analog input, and other designated pins can be PWM outputs.
Also I programmed the un-used PWM pins as INPUT_PULLUP so I did not have to install pull up resistors. Also the reset pin is not connected and it is using its internal pull up resistor. Why not use the reset pin? It is not needed, the compiled code using Aduino C++ has the ATMega ATtiny 85 in a tight loop it cannot get out of program.
I thought of using the un-used outputs to light status LEDs but that would involve un-necessary parts for operation.
Why did I use the ATtiny 85 instead of the ATtiny 45? The 85 cost was cheaper than the 45 when I bought two, the one I used in this project has been re-programmed many times it is ready to do something else even if it is constantly polling for an Off hook condition.
This project has an AC power cord and a low voltage barrel jack for the 9 volt DC to power up the electronics. A final product would have one AC power cord with a switching supply inside its enclosure.
As I pointed out earlier this project would be better designed into a Cordless Phone, using software, to reset the OFF HOOK phone, and add a do not disturb function, while silently recording messages.