What is RotaryCell?

RotaryCell is a reversible internal conversion that gives a traditional rotary telephone its own cellular connection. The finished phone operates much like it originally did: lift the handset for a dial tone, dial with the rotary wheel, and answer incoming calls when the original mechanical bells ring.

The design preserves the original dial, handset, switchhook, network block, ringer, wiring, and case. The added hardware fits inside a Western Electric 500-series telephone without drilling or cutting it.

System architecture

A LilyGO T-SIM7670G-S3 combines an ESP32-S3 controller with an A7670G LTE modem. The ESP32 reads the switchhook and rotary dial, generates telephone tones, controls cellular calls, and manages the other phone functions.

A Silvertel AG1171 subscriber-line interface connects the modern electronics to the telephone’s original analog circuitry. It provides the line voltage, supervises the switchhook, carries handset audio, and supplies the ringing signal for the mechanical bells.

Two custom circuit boards complete the interface:

• The AG1171 carrier connects the line-interface module to the telephone, controller, and power system.
• The Audio/Reset A4 board conditions microphone and speaker audio and provides a hardware power-cycle circuit for recovering the cellular modem.

Power and packaging

The phone runs from a rechargeable Samsung 58E 21700 lithium-ion cell. A USB-C connector installed in the telephone’s original rear opening supplies charging power. Custom printed carriers secure the LilyGO board, custom PCBs, and rear connector inside the housing.

A typical charge provides roughly two days of operation, depending on signal conditions and use.

Firmware

The firmware handles:

• Rotary pulse decoding
• Switchhook detection
• Incoming and outgoing cellular calls
• Dial, reorder, and receiver-off-hook tones
• Mechanical bell ringing
• Battery monitoring
• Persistent event logging
• Browser-based maintenance and diagnostics
• Web firmware updates
• Manual hardware modem recovery

Building one

The project is fully documented. The illustrated guide covers sourcing the major components, ordering the custom PCBs, printing the mounts, preparing the LilyGO board, assembling the wiring, installing everything in the telephone, loading the firmware, adjusting the audio, and testing the completed phone.

Illustrated build guide:
https://evilroot.net/RotaryCell/

Firmware, PCB files, printable parts, and engineering documentation:**  
https://github.com/fregacmols/RotaryCell

Demonstration video: