From FB group Bell Telephone…
a reply to Lucky225
I fixed the same problem. A single 1N4003 rectifier diode in series with the LED. If you think there’s too much flicker, add a 220 uF capacitor across the LED. The LED must have a resistor in series, start with 1000 ohms (brown, black, red, gold stripes). Here is a photo. The yellow LED is lit, it doesn’t look bright. If you connect the LED backwards it won’t light, just try it the other way. All the parts are available at an electronics store (if you can find a Radio Shack). The rectifier might come in a package of a few and cost a dollar or so. Same with the resistor. You can order them on eBay for cheaper, free shipping, but it takes a week or two from China. So use the original Ma Bell Trans, with the screw and holder clip.
I forgot to say, you don’t have to solder the parts. You can connect them with crimp-on ‘beans’ or ‘mouse condoms’ or twist the wires, and cover the bare wires with tape or heat shrink tubing. You could put the parts inside of the phone. The color of the LED isn’t important.
