Editorial disclosure: I work at Zellbox. Cal.com steps described from their Google Calendar app docs as of August 2026.
Cal.com is the open-source scheduling primitive. Zellbox layers on the piece Cal.com doesn’t do: WhatsApp reminders. The bridge is your Google Calendar.
How it works
- In Cal.com → Apps → Google Calendar → install and pick as destination calendar.
- In Zellbox → Onboarding step 2 → connect the same Google Calendar.
- Every Cal.com booking now lands on your calendar. Zellbox fires WhatsApp reminders at 24 h and 2 h automatically.
What Zellbox adds beyond Cal.com’s built-in workflows
- WhatsApp reminders on top of Cal.com’s email + workflow-triggered SMS.
- Language-aware templates for multilingual customer bases.
- Signed consent PDFs — patients sign on their phone; PDF stored on the customer record.
- Two-way replies — customer responses land in your Zellbox inbox, not lost to the void.
Feature summary
| Zellbox layer | Cal.com built-in | |
|---|---|---|
| Booking + rescheduling | via Cal.com | Native + open-source |
| Email reminders | Not the focus | Native |
| Workflow-triggered SMS | Not the focus | Native (paid) |
| WhatsApp reminders | Native + free tier | Not available |
| Signed consent PDFs | Native | Not available |
Setup ~10 minutes end-to-end — no BSP contract, no code.
About the author: Nacho Coll is the founder of Zellbox. He writes about WhatsApp Business automation for appointment-driven small businesses.
