# 5.4 Tips & troubleshooting

- **Customer with no contact:** a customer must have **phone or email** (at least one). With no
  contact, the appointment still saves, but the customer isn't created/linked and gets no
  notifications.
- **Phone numbers:** stored in international format (E.164). Use the country selector in the phone field
  to get the right dial code.
- **Customers aren't shared across businesses** — each business has its own base.

> 💡 **Scenario:** a customer booked online with their email and later called and booked by phone. The
> system detects the **duplicate** (same name/contact in different records) and shows the badge — do a
> **merge** (5.2) to keep one unified history.

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**Next:** [Messages & notifications](06-messages.md).