I used those to call back to the UK when I moved to the US, back around 2005.

When you entered the number you wanted to call, it would do a VOIP connection from the line you were on to that company to a line in the destination county. So it was an Internet call for the international part (which is how they did it cheaply).

I realized that because the cards I bought were from a company with “VOIP” in the name!

I had one of those Radio Shack tone dialler boxes so that I could pre-program the free US number and the card id number.