An open-source board combines an ESP32-S3, cellular modem and telephone interface circuit to let an unmodified rotary phone make and receive mobile calls.
An open-source project called RotaryCell puts an unmodified rotary-dial telephone onto a cellular network. The design combines an ESP32-S3 microcontroller with a cellular modem and the line interface hardware normally provided by a telephone exchange. The project’s code and documentation are published under the MIT licence, while schematics and third-party datasheets retain their respective original ownership.
A rotary telephone is not simply a microphone and speaker. It expects a telephone exchange at the other end of its two wires. That means a direct-current line voltage to support the telephone circuit, a low-frequency alternating ringing signal to operate the mechanical bell, and equipment capable of detecting and counting the pulses generated by the rotary dial. Modern cellular networks provide none of these functions directly.
RotaryCell supplies these functions locally. A Silvertel AG1171 subscriber line interface circuit (SLIC) provides the telephone line interface, including line feed, ringing functions and hook-switch detection. The ESP32-S3 detects and counts the pulses generated by the rotary dial and converts them into dialled digits. A LilyGO T-A7670G-S3 board combines the ESP32-S3 with a SIMCom A7670 cellular modem, which connects the telephone to the mobile network and handles the call. A separate board conditions the audio between the modem and handset, with adjustable signal levels in both directions.
The firmware also generates call-progress tones locally, allowing the telephone to behave more like it did on a conventional landline. It provides dial tone, reorder tone and a receiver-off-hook warning based on the North American telephone plan. The unit runs from a protected 21700 lithium-ion cell with battery monitoring, synchronises its clock from the cellular network, and supports over-the-air firmware updates, USB diagnostics and a maintenance Wi-Fi access point.
Firmware version 0.10.4 had been tested on a working prototype as of August 2026. Custom printed circuit boards had been ordered but were not yet validated, so the author describes the design as pre-production rather than a finished product. Users attempting to build the project should therefore expect some hardware debugging and validation work.
The interesting part is not the nostalgia. A SLIC combined with a cellular modem can turn a conventional two-wire analogue telephone into a cellular endpoint without modifying the telephone itself. The same approach could potentially support applications such as lift emergency handsets, intercoms, factory ringdown phones or private branch exchange extensions left without service after a landline is disconnected. In such installations, replacing the existing telephone infrastructure can also require changes to wiring and associated equipment.
India’s fixed-line subscriber base has declined for decades while analogue handsets remain installed in homes, offices, lifts and small factories. A cellular bridge could keep such hardware working instead of replacing it. Sourcing is relatively straightforward: the SIMCom A7670 family and similar cellular modules are available through electronics suppliers, while LilyGO boards can be obtained through import channels.
Two changes would be needed for Indian use. The firmware currently follows a North American tone plan and would need to be adapted for Indian call-progress tones and ringing conventions. Any version developed as a commercial product would also need to comply with the applicable Indian telecom and wireless equipment regulations, including requirements that may apply to the cellular radio and the finished telecom equipment depending on its final classification and configuration.
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