Use the confirmed booking as the source
Read the appointment identifier, date, time zone, location and owner from the booking system. Keep confirmation separate from later reminders. An unconfirmed slot must not trigger a message that presents it as a completed booking.
Make changes update the reminder sequence
When an appointment moves, cancel the old scheduled reminders and use the new details. Stop the sequence after cancellation. Decide who receives a rescheduling request and how the newly available slot returns to the calendar.
Choose a channel the customer can use
Use WhatsApp, RCS, SMS or voice only where approved for the customer and market. Include one useful next step rather than a second sales pitch. A fallback route needs its own approved timing; it must not override an opt-out. Compare the available contact channels.
Measure attendance independently of delivery
Track reminders sent, delivered where reported, replies, cancellations, rescheduled appointments and attendance as separate outcomes. A delivered reminder does not prove the customer read it or attended. Compare attendance for equivalent appointment groups and allow the appointment date to pass before judging the result.
Will reminders prevent every no-show?
No. Availability, changing needs and practical circumstances still affect attendance. Make it easy to change the booking and measure the outcome in your own appointment system.