All these cool projects have helpful information about interfacing with phones. Thanks for the inspiration!

Bluetooth Rotary Phone - Nathan Seidle, 2005

Interface a Rotary Phone Dial - guidomax, 2009

Simple Phone Intercom - Jason Poel Smith, 2012

Arduino Rotary Phone - Andrew Stella, 2014

Cellular conversion of vintage rotary phone - Johan Berglund, 2016

Rotary Dialer to USB Keyboard - Max Romagnoli, 2017

A Strange Phone - Guy Dupont, 2022

Touch Tone Dial-A-Song - John Park, 2022

Intercom System from Rotary Telephones  - Thomas, 2023