I was thinking that there should not be any huge technical problem with making an FXS interface for the raspberry pi, and I did find a USB adapter which I might have been tempted to to try, but it's a little expensive and only supported 0.7 REN (ringer equivalency number) which might not be enough to power the mechanical ringer on an old phone. Further searching did turn up the OAKR2 HAT though ( https://switchpi.com/oakr2-module-specifications/ ) and it looks like the schematics are on github.