One blob of solder across the USB-OTG pad on the back of the board, near the USB-C ports. This is the only soldering. It lets the native USB port send 5 V out to power the handset. The IN-OUT pad next to it does nothing for the handset.

USB port: the handset, and only the handset. COM port: the computer (for flashing) or the charger (every day).

Never plug a computer into the USB port once the pad is bridged: that port now pushes 5 V out.

Switch tabs: GPIO 4, 5, 6, 7 to GND, one per person. The switch is polarity-free.

Lamp tabs: GPIO 15, 16, 17, 18 to GND, no resistor (the 12 V lamp has its own and still glows at 3.3 V). The lamps are polarized; if one stays dark, flip its two wires.

A plain LED works too: pin, 330 ohm, LED, GND. Buttons you do not wire are ignored.

1. Sign up at twilio.com.
2. Upgrade from the trial (about $20 credit). A trial plays a message on every call and can only call verified numbers.
3. Fill in your customer profile (Trust Hub). Accounts created outside the US/Canada need a business profile to call +1 numbers (error 13225).
4. Buy one US number with Voice.
5. Turn on Geo permissions for your family's countries.
6. Set a usage alert and leave auto-recharge off.
7. Keep the Account SID and Auth Token handy.

Open allanbinder.github.io/kid-phone in Chrome or Edge with the computer on the COM port. Press Connect, pick the port, Install. Then type your WiFi name and password when the installer asks. If the board does not show up, hold BOOT while plugging in.

Open kidphone.local (or the IP the installer showed). Add people with full +country-code numbers, paste the Account SID and Auth Token, press Connect. In about a minute the phone creates its SIP login, the SIP domain, the allowlist Function and a restricted key, then forgets the Auth Token. Optional: call hours per person in their own time zone, and a bedtime window.

Handset into the USB port. Pick it up and press a lit button. If something is wrong, the phone says what in a short spoken sentence, and the setup page shows the detail.

A shoebox works: holes in the lid for the buttons, a photo of each person above their button. Zip-tie the handset cord inside the box for strain relief so a yank pulls on the box, not the USB socket. Wall mounting at the kid's height is recommended: a toddler running off with the handset will find the end of the cord.