TL;DR: The Twilio helper library is not installed for your Python. Run python -m pip install twilio with the same interpreter that runs your code.

```text
ModuleNotFoundError: No module named 'twilio'
```

## Fix it

1. Install: python -m pip install twilio. Expected: Successfully installed twilio.
2. Verify: python -c "from twilio.rest import Client; print('ok')". Expected: ok.
3. If it still fails, align pip and python: python -m pip --version must match which python. Expected: same interpreter.
4. Add twilio to requirements.txt.

## When this applies
- import twilio fails with ModuleNotFoundError.

## When it doesn't
- The import works but TwilioRestClient fails: that is the v6 rename, different skill.
- API calls 401: credential problem, not this.

## Compatibility
- twilio 7.x/8.x; Python 3.8+.

## Why it happens
Fresh venvs and containers start empty; nothing implies twilio is installed.

## Edge cases
- The package is twilio on PyPI but some old guides say pip install twilio-python; both resolve, but twilio is canonical now.