Phone Recorder is an application for Windows 10
that allows you to join and record an ongoing telephone
conversation. It runs under either 64-bit or 32-bit Windows.
Phone Recorder doesn't have much of a user interface — just
two buttons: "Start recording/Stop recording" and "Exit".
When you click on "Start recording", Phone Recorder will join
an ongoing phone conversation on the same line as that of the
modem, and the button label will change to "Stop recording".
The conversation is stored as a .wav file in the same folder
as the Phone Recorder executable (.exe) file. To run Phone
Recorder, you need to have a dial-up modem that has voice
capability installed on your computer. Phone Recorder uses
only the voice feature of the modem — there's no need to
"dial-up" anywhere. Be sure to tell the other party or
parties on the line that you are recording the conversation
if it is legally required to do so.
Here's a link to the Phone Recorder app kit: