I brought this phone back to life with a playlist of recordings by my niece's family and winnie the pooh and other sound tracks...

An SD card has the sound files that are played into the original earpiece when the three digit phone number is dialed.

Required Parts:

rotary phone from goodwill $15
teensy uController $20
teensy audio shield and SD card ~$20
mosfet, resistor, diode, and LED (see next step)

Step 1: Hack Your Phone

Rotary Dial Hack video

You have to determine which wires are needed.

There's a bundle of wires coming out of the bottom of the dial. I used the green and blue wire. The blue wire was ground(ish) and the green wire pulsed (LOW) once (when you dialed ONE), twice (when you dialed TWO), three times(when you dialed three), and so on.

I used a mosfet to trigger an LED and a digital INPUT to the teensy to read the pulses.

Any signal, N-channel mosfet will work. I used a generic mosfet like this. You may also try a transistor. Here's a link to an assortment. You also need a diode to rectify the pulses as they are slightly AC*. Any diode will probably work.

If you want to the phone hook, so that when you lift up the phone, the dial sound starts, you need to identify the two wires that are connected to the hook switch and run one to ground and one to another Digital Input on your teensy.

* I didn't put a diode on the ground line to fully rectify the AC pulses coming out of the dial. I forget why but it seems to work OK so far.

Step 2: Finish Your Wiring

I connected the wires from the original rotary phone earpiece to the teensy audioi shield 1/8" jack. The audio shield has an onboard amplifier that has plenty gain and LOUD as you want it for an small handset earpiece.

I wired the mosfet and resistor into a small protobaord. You don't have to leave the LED in place but by doing so it shines through the clear acrylic (see the video) and looks pretty cool. You could add a second LED so their are two lights under each lift.

I only have two digital inputs. One for the rotary dial and one for the lift on the handset (know when the phone is picked up).