Rotary phones where used with https://en.wikipedia.org/wiki/Strowger_switch - so they have very simple interface - just generate the number of pulses that is either N or 10-N (I don't remember exactly which one is right). There is also a nice back-story about how it was invented.
This is a nice DIY project, but its practicality is questionable. Choosing a number on rotary phones is very slow, as you need to wait til the wheel returns to its start position.
You could redesign the rotary dial to return to zero much, much faster with shorter pulses and smaller intervals between them given advances in high frequency signal counting.
I remember adjusting my phone when I was a kid to spin back as fast as possible, if set too fast and the number wouldn't dial. There was a little speed governor inside which could be adjusted for more or less resistance, i think it had weighted arms that could be adjusted outwards to slow it down.
This is a nice DIY project, but its practicality is questionable. Choosing a number on rotary phones is very slow, as you need to wait til the wheel returns to its start position.