RotaryCell turns an unmodified rotary phone into a portable LTE handset

A classic desk phone gets a cellular brain

A project that made the Hacker News front page lays out how to convert a traditional rotary telephone into a portable, battery-powered cellular phone without altering the original hardware. The repository, published as RotaryCell on GitHub by the user fregacmols, keeps the telephone's handset, rotary dial, switch-hook, mechanical ringer and network block in service, with all added electronics mounted reversibly inside the case.

According to the repository, no original part needs to be drilled, cut or permanently changed. The result is a desk telephone that can be carried away from any fixed line and used at meetings, demonstrations or anywhere compatible cellular coverage exists, while remaining fully restorable to stock condition.

How the hardware works

The core of the build is a LilyGO T-A7670G-S3 Standard board, which combines an ESP32-S3 microcontroller with an A7670 cellular modem, battery charging circuitry and a cellular audio interface. A Silvertel AG1171 subscriber-line interface chip operates the telephone's line circuitry, senses when the handset is lifted, and drives the mechanical ringer.

A separate Audio and Reset A4 board adds adjustable transmit and receive audio conditioning plus a hardware power-cycle circuit that can recover the modem when software-only resets are not enough. Power comes from a single protected 21700 lithium-ion cell wired to the LilyGO's battery pads, replacing the phone's original 18650 holder and also supplying the AG1171 directly.

One repurposing stands out: the telephone's RJ11 line jack is used only to deliver regulated 5 V to the LilyGO's charging input on designated pins. It is not a telephone-line interface, and the repository warns never to connect the prototype's Tip/Ring wiring or the repurposed jack to the public telephone network or energized premises wiring.

Firmware features

Firmware v0.10.4 is the current prototype-tested release. It implements rotary pulse dialing and switch-hook detection, incoming and outgoing cellular calls, physical bell ringing through the AG1171, and North American dial, reorder and receiver-off-hook warning tones. Handset audio works in both directions with adjustable levels, and the firmware adds battery monitoring, USB diagnostics, a temporary maintenance Wi-Fi dashboard, a browser and USB AT-command terminal with a persistent event log, cellular clock synchronization and over-the-air application updates.

Dialing the service code 0000 starts the maintenance Wi-Fi access point, while 9999 runs modem diagnostics and software recovery. The repository notes, however, that the latter failed to clear a field-observed modem lockup that is tracked in STATUS.md.

A working prototype, with PCBs still pending

The repository is explicit that it records work in progress rather than serving as a finished construction kit. As of August 28, 2026, the hand-wired prototype operates, while two custom boards had been ordered from JLCPCB — the Audio and Reset A4 PCB on August 27 and the AG1171 carrier on August 28 — and are archived exactly as submitted, awaiting assembly and validation.

For anyone recreating the proven prototype by hand, a full-resolution printable PDF documents the complete point-to-point wiring across the LilyGO board, the 21700 cell, passive audio components, the AG1171, GPIO assignments and the original Model 500 circuitry. The repository cautions that fitting and insulating loose components inside the telephone case is challenging, and builders should expect several rounds of test-fitting and careful wire routing. Code and original documentation are MIT-licensed, and the default maintenance Wi-Fi password is published as rotarycell, which builders are told to change before using the device around untrusted people.

Why it matters

Most retro-phone conversions require cutting into or gutting the original hardware. RotaryCell demonstrates a fully reversible path that keeps a collectible telephone intact while making it genuinely usable on modern networks — pulse dialing, mechanical bell and all. The documentation discipline also sets a useful example for open hardware projects: it clearly separates tested behavior from unvalidated boards, records a known failure mode, and publishes exact as-ordered manufacturing packages. That is precisely what other builders need to reproduce or extend the work rather than starting from guesswork, and it is the difference between a demo and a durable engineering baseline.