Best SIM Card for a Raspberry Pi
AI Agent or Hardware AI Build

A Raspberry Pi running a local LLM or agent framework is a capable, low-cost compute platform for autonomous tasks. But hardware AI builds hit the same wall every cloud-based agent does: SMS verification. This guide covers the complete setup for adding cellular connectivity to a Pi-based AI build, including SIM selection, USB modem configuration, and integration with common Python agent frameworks.

Why Physical SIM on a Pi Is Different from Cloud

On a cloud VM, a USB modem isn't an option — there's no physical USB port. On a Pi, it is: the four USB-A ports on a Pi 4 or Pi 5 will accept any USB LTE modem directly. This makes the Pi the ideal platform for agentic systems that need real SMS capability, because the hardware path from SIM to agent code is as short as it gets.

The SIM you choose matters. Consumer SIMs tied to carrier accounts introduce identity exposure and registration requirements that defeat the purpose of a private agent build. A KYC-free SIM — ordered without any ID requirement — gives you cellular connectivity with no identity trail attached to the number.

Choosing a USB Modem

The Pi's USB ports run USB 3.0 on Pi 4 and Pi 5. Most USB LTE modems are USB 2.0 and will work fine. Three reliably Pi-compatible models:

Huawei E3372h — The most widely tested. Plug-and-play on Raspberry Pi OS. Appears as both a network interface and a serial device. Watch for the firmware variant: the "h" suffix models expose a CDC-Ethernet interface that works without drivers; the "s" models require HiLink mode switching.

ZTE MF833V — Slightly smaller form factor, similar compatibility. Enumerates as /dev/ttyUSB0 immediately on Pi OS without additional configuration.

Quectel EC25 — More commonly found on Pi HATs designed for cellular. If you prefer a HAT form factor that sits cleanly on the Pi's GPIO header, the Waveshare SIM7600 series uses a Quectel module and is well-documented for Pi OS.

Initial Setup on Raspberry Pi OS

Insert the SIM, connect the modem, and boot. Check that it's recognised:

lsusb
# Look for your modem manufacturer (Huawei, ZTE, etc.)

ls /dev/ttyUSB*
# Typically ttyUSB0, ttyUSB1, ttyUSB2 — each is a different interface

Test the AT command interface on the first port:

sudo apt install minicom
minicom -D /dev/ttyUSB0 -b 115200

# In minicom, type:
AT # Should respond: OK
AT+CIMI # Returns IMSI — confirms SIM is read
AT+CSQ # Signal quality: 99,99 means no signal; 10-30 is normal

If signal quality is low, USB extension cables can help position the modem for better reception, especially indoors.

Configuring SMS Receipt

AT+CMGF=1 # Text mode (not PDU)
AT+CNMI=2,2,0,0,0 # Push new SMS to serial immediately
AT+CPMS="SM","SM","SM" # Use SIM card storage

Send these commands once after each modem power-on. In production, add them to your modem initialisation function so they're set automatically when your agent service starts.

Running as a Systemd Service

For a Pi that runs unattended (headless, always-on), wrap your agent as a systemd service so it starts on boot and restarts on failure:

# /etc/systemd/system/simbotica-agent.service
[Unit]
Description=AI Agent with SMS Capability
After=network.target

[Service]
ExecStart=/usr/bin/python3 /home/pi/agent/main.py
Restart=on-failure
RestartSec=10
User=pi

[Install]
WantedBy=multi-user.target

sudo systemctl daemon-reload
sudo systemctl enable simbotica-agent
sudo systemctl start simbotica-agent

Power and Thermal Considerations

USB modems draw 300–800mA during active data transfer. On a Pi 4 running a local LLM with a modem attached, total draw can approach 3–4A. Use the official Pi power supply (5V/3A minimum) or a powered USB hub for the modem. Thermal throttling under sustained LLM inference load can affect serial reliability — a heatsink or active cooling on the Pi CPU is recommended for always-on builds.

Integrating with a Local LLM Agent

If you're running Ollama, llama.cpp, or a similar local inference server alongside your agent, the SMS reader integrates cleanly as a tool. The function call from the agent to the SMS reader is local (no network hop), which keeps latency low even on Pi hardware. A typical agentic loop on a Pi 4 with a 7B model: plan, act on web, hit SMS wall, call receive_sms_otp(), continue — end-to-end without cloud APIs.

Multiple SIMs on One Pi

A powered USB hub lets you attach multiple modems to a single Pi. Each modem appears as a separate set of ttyUSB devices. Assign one modem per agent thread and the isolation is clean. Simbotica sells up to three SIMs per order — a natural fit for a three-thread agent setup on a Pi with a four-port hub.