Original Contents of file notes.txt as contained in file "Pulse2Tone_2012_01_22.zip"
(Hyperlinks added by JvK) (The PDF-file only contains a picture, which is directly stored as JPG in this repository)

main.c - source code
main.h - header file
Schematic_v1-0.png - adapter schematic
wd_we500c-d.pdf - phone schematic

Source code compiled with AVR Studio v4.18 + WinAVR for attiny45, optimization -O1.
SUT_CKSEL fuse should be programmed for external 8 Mhz crystal oscillator.

The converter originally designed to be mounted/wired inside the phone. 

It has to be connected in parallel to the phone line, but _after_ the hook switch, e.g. between terminals C and F/RR on the phone schematic. 

Rotary dial (all 4 wires) has to be disconnected from the phone and wired to the adapter (see schematic). 

The adapter is polarity sensitive.

Terminal C is just a solder pad, so I ended up using L2 instead (it’s a nice screw terminal).