gsm-modem

hum-on-a-burner-phone — USB GSM modem bee driven by AT commands

A bee that turns a cheap USB GSM dongle into a hum agent. SMS in → chi:"prompt" to humd → daemon’s reply sent back as SMS via AT+CMGS. No carrier API, no webhook, no public IP — the modem talks to the cellular network directly.

Same conversation continues across messages from the same number (sid = sha256(“gsm-modem:From”)[..16]).

Propensity

statefulness	richness	wire shape	hides
stateful (per-phone sid)	lean	GSM AT-command serial (`/dev/ttyUSB*`)	tools, system prompts, perf, drone, breath

Wire

SIM card phone ──SMS──► cellular tower ──► your USB modem
                                                │
                                                ▼ +CMT URC over serial
                                          gsm-modem bee
                                                │
                                                │ chi:"prompt"
                                                ▼
                                                humd
                                                │
                                                │ chi:"chunk" → "finish"
                                                ▼
                                          gsm-modem bee
                                                │
                                                ▼ AT+CMGS over serial
                                            USB modem ──► tower ──► phone

Tested modems

The AT command set used here is the GSM 07.05 SMS subset. Anything that speaks SMS text mode works:

modem	port style
Huawei E303 / E353 / E3531	`/dev/ttyUSB0` (the AT port — others are NMEA/diag)
SIMCom SIM800 / SIM900 / SIM7600	`/dev/ttyUSB0` (HAT-style boards via USB-UART)
Quectel EC25 / EG91	`/dev/ttyUSB2` (AT port; check vendor docs)

A SIM card with an active SMS plan is required. Prepaid burner SIMs work fine.

Configure

env	default	what
`HUM_GSM_DEVICE`	`/dev/ttyUSB0`	serial path the modem appears on
`HUM_GSM_BAUD`	`115200`	baud rate
`HUM_GSM_MODEL`	`claude-haiku-4.5`	model humd spawns
`HUM_GSM_SYSTEM`	terse SMS-friendly system prompt	system instruction
`HUM_GSM_REPLY_LIMIT`	`1500`	hard cap on reply length
`HUM_THRUM_SOCK`	`$XDG_RUNTIME_DIR/hum/thrum.sock`	humd’s NDJSON socket

The process needs read+write access to the serial device. On Debian / Ubuntu add yourself to dialout:

sudo usermod -aG dialout $USER
# log out + back in

Install

npm install
npm run build
npm start

serialport is a native module — needs build tools on systems without prebuilts. On Debian: sudo apt-get install build-essential python3.

Test without a SIM

You can pipe AT commands at the bee via a fake serial pair using socat:

sudo apt-get install -y socat
socat -d -d pty,raw,echo=0 pty,raw,echo=0
# socat prints two /dev/pts/N paths — point HUM_GSM_DEVICE at one,
# then write +CMT URCs to the other to simulate inbound SMS.

What flows where

modem URC / AT	hum chi
`+CMT: "<from>",...\n<body>`	`chi:"prompt"` (sid keyed off `<from>`)
daemon’s `chi:"chunk"` text parts	collected into one SMS body
daemon’s `chi:"finish"`	`AT+CMGS="<from>"` + body + Ctrl-Z

What it doesn’t do

No PDU mode. Text-mode (AT+CMGF=1) only — fine for plain ASCII / GSM-7. Emoji and non-Latin characters need AT+CSCS="UCS2" + PDU-encoded payload, which this bee does not implement.
No long-SMS concatenation. Replies > 160 chars are clipped at HUM_GSM_REPLY_LIMIT. A future revision can split into multi-part SMS via UDH headers.
No call handling. Voice (RING URC, ATA / ATH) is not wired. Easy to add — answer with ATA, hang up with ATH, but the audio path requires a separate codec stack.
No SIM PIN unlock. If your SIM requires a PIN, send AT+CPIN=<pin> manually before starting the bee (or extend init()).

Sibling: DECT

DECT (cordless home phones) is a related radio standard but practical programmatic access is poor — most DECT base stations expose proprietary HTTP/RPC and a custom firmware is needed to drive the radio directly. If you have a Gigaset SL series base station or an open-DECT hardware project (rare), the wire mapping would be similar to this bee but the transport is HTTP, not serial.