You are helping me build the following AI agent workflow.

## Goal
WhatsApp Booking and Cal.com: Inbound WhatsApp messages trigger an AI assistant that books appointments via Cal.com, logs them to Sheets, and sends SMS reminders.

## Specification
- What it does: You receive instant booking confirmations and timely SMS reminders when clients schedule appointments directly through WhatsApp. This process automatically handles your scheduling requests by checking availability, logging details to Google Sheets, and managing the entire conversation without manual intervention.
- Trigger: Triggered by an event (Event · on new WhatsApp message)
- Autonomy: Fully hands-off
- Expected setup effort: about an afternoon
- Tools/services involved:
  - WhatsApp API: Inbound message trigger and confirmation delivery
  - OpenAI Chat Model: Conversation flow and qualification via GPT-4
  - Cal.com API: Fetching slots and booking appointments
  - Google Sheets: Storing appointment details and status
  - SMS Service: Sending pre-appointment reminders

## Known pitfalls, handle each one explicitly in your implementation
1. WhatsApp requires strict template approval for outbound messages so you must pre-approve your confirmation message templates before going live.
2. Cal.com API calls can fail if timezones are mismatched, so always normalize timestamps to UTC before sending requests.
3. Phone number formatting varies across regions, so use a library like libphonenumber to standardize inputs before storing them in Google Sheets.

## Reference implementation
https://n8n.io/workflows/4949 (workflow template)
Fetch and inspect this before building. If it matches my stack, adapt it;
if not, rebuild the pattern with my tools.

## Process requirements
1. Before building: ask me which of the listed tools I actually use and
   what my platform is (n8n / Make / code / other). Do not assume.
2. Adapt the pattern to my answers; do not force the reference stack.
3. Address every pitfall above; tell me how you handled each.
4. Provide a test plan I can run before letting this touch real data.
5. Ask before any step that sends messages, modifies data, or spends money.

Source: https://usecasesforagents.com/use-case/automate-whatsapp-booking-with-gpt-4-and-cal-com/ via usecasesforagents.com