Open-source USB-C LTE modem gives computers voice, SMS and data for scripts and agents

A phone line over one cable

A project published on GitHub by Koppar and surfaced on Hacker News lays out a four-layer circuit board that gives a computer its own cellular line through a single USB-C connection. Insert a nano-SIM, plug in, and the machine can use LTE data, send and receive text messages, and place or answer voice calls with the audio played through the computer's headset. Everything needed to build it — the KiCad 10 project, Gerbers, JLCPCB assembly files, a schematic PDF and a written design review — is published under the CERN Open Hardware Licence, version 2, Permissive.

What the computer sees

According to the project's README, the board's on-board USB hub presents three ordinary devices to the host. A network adapter, backed by a Quectel EC25-EUX module, provides LTE data with fallback to 3G and 2G on European bands. Serial ports expose standard AT commands, so software can poll caller ID, send texts and answer or end calls. And a USB sound card built on a Raspberry Pi RP2040 microcontroller takes the modem's 1.8 V PCM interface, level-shifts it, and exposes it to the computer as a microphone and speaker pair — which is how live call audio reaches the host.

There is no battery, screen or speaker on the board itself; three LEDs indicate network registration, an incoming ring and an active call. Power comes from USB-C at 5 V, stepped down by a 3.8 V, 3 A regulator sized for the modem's 2 A transmit bursts, plus a 3.3 V rail for the hub and microcontroller.

Built to be driven by software

The split between control and audio is the point. Calls are managed over serial AT commands while their audio arrives as a standard USB sound device, so a script or an AI agent can answer the phone, listen, speak and hang up without any telephony stack on the host. The README pitches the board as a way to hand calls and texts to programs rather than people.

Design review, sourcing and caveats

The board is at revision B. The published review records that revision A had an antenna feed at 35 ohms that changed layers, a buck converter placed 60 mm from its inductor, and a SIM socket facing the wrong way; all three were corrected. Revision B reports zero DRC errors and zero unconnected nets in KiCad 10, and the boards are being ordered.

Several parts must be sourced by the builder. The modem, ordering code EC25EUXGR-128-SGNS, is not fitted by the assembly house and must be soldered separately — and it has to be the EUX variant, because the North American data-only versions of the module lack voice support. Two U.FL antennas covering 698–2690 MHz go on the main and diversity connectors, and the SIM needs a plan that includes voice. The cable matters too: the README warns that a USB-A port or any 1.5 A source will brown out during a 2G transmit burst, so the board needs a port supplying 3 A at 5 V.

Recovery paths exist for when things go wrong: pads to force the RP2040 into its USB bootloader even after crashing firmware, an SWD header, and a pad pair that puts the EC25 into emergency download mode after a failed update. The modem does not switch itself on — the RP2040 drives its power and reset lines — so until firmware runs, the computer sees only the hub and the microcontroller.

The project is not yet proven hardware. Bring-up, the AT command work, the sound-card firmware and the first call and text are all listed as next steps once the boards arrive. The README also notes that VoLTE is optional on this module: in regions where 2G and 3G networks are switched off, calls require a modem firmware with VoLTE and a carrier that permits it.

Why it matters

Most computers reach the phone network through apps, browser-based numbers or SMS gateways, each with its own account layer and API limits. This board makes a carrier phone line behave like three plain peripherals — a modem, serial ports and a sound card — a form factor scripts and AI agents can use directly: answer a call, process the audio, reply by text, escalate to a human. The transparency is unusual for hardware too; the design review documents the first revision's mistakes with numbers attached, and the whole package is licensed for reuse. Finally, the board doubles as a demonstration of its designer's service: Koppar says it produced the board from a one-page text specification and offers both spec-to-board design and an MCP interface so a customer's own agent can drive the flow.