This software is for an arduino pro micro (atmega32u4). It acts as an USB keyboard and converts impulses from a rotary disk into digits transmitted via USB to any keyboard supporting software.
Impulse contact (nsi) is connected to pin 10 and nsa to pin 9. nsa gets closed
as soon as the disk is revolved. nsi starts generating impulses as soon as the
disk is released and rotates back.
Needs only 3 wires: nsi, nsa, and 1x ground to the 2 switches. Internal pull-up is used
for pin 9 and 10.
Uses TimerOne library. Search for how-to tutorials if it is your first arduino library you install.
If you connect it to your computer (OS does not matter, as long as it supports USB HID), you might get an error message. This is for the programming serial line, also implemented on that USB device. You can ignore that, after ..9 seconds rotary disk keyboard should be ready.
And if you think twice, impulse dialing is digital dialing. Since 1913 (or so)