I have added a switch to choose between a 620 ohms and 33k resistor. This
attenuates the signal coming out from the interface, provided there is
at least a small load(10k works fine) between audio out and audio gnd.
Important: This circuit works in Canda and probably in the United States. It has not been tested in europe, but
it may work if the phone system is similar enough.
Here are 2 pictures showing the outside and inside of my interface:
All the components I used are from an old answering machine, except the
case, which I bought at King George electronic.
I'm not responsible if you use this to invade privacy of people.
I cannot be held responsible for any damages that could occur to you
or your equipment while following the procedures present on this page.
Also, I GIVE ABSOLUTELY NO WARRANTY on the correctness and usability
of the informations on this page. Please note, however, that the procedures
above have worked in my case without any damages or problems.