The video shows my most recent portable phone, a Belgian RTT model 56 in black.

To show you the internals, an older version is opened. This Belgian phone was made by the Bell MFG. Company.

Other previous versions include:

Version 1:

A HEEMAF model 1955.

The electronics were built around a Siemens M35, which accepted AT commands.

Instead of generating 100VAC for the ringing circuitry, I scrapped the ringer and used a hard drive read arm to ring the bells. I wired 3 AAA batteries instead of the battery pack.

Version 2:

A Belgian RTT model 56 (like the one in the video, but with copper coating).

This version was bluetooth-based and connected to my Nokia using an NXP BGB203 Bluetooth SiP with SPP firmware. A bluetooth headset was integrated in the horn and powered through the cord.

In this version, I integrated a 50V step-up converter for the first time, inspired by Sparkfun's discontinued blue rotary phone.

This worked nicely, but I wanted the phone to be completely stand-alone, and modern smartphones generally don't send and accept AT commands over bluetooth serial port anymore.

Version 3 and 4:

See the video.


  • Previous versions included a "travel mode" that made it possible to pack the phone with the horn off the hook and still be reached. Still have to implement this.
  • Currently, the phone awaits 10 digits before calling, restricting it to 10-digit numbers. Also it cannot dial star or hash.
  • A short "warning" ring to indicate that the battery is running low.
  • Still have to find a proper antenna solution that can stick on the underside of the body.