Your customers may still call you, but they don't have to call to cancel or reschedule an appointment.

The prospect can always reschedule or cancel the appointment directly from the confirmation email or the appointment reminder email. 

The prospect can also reschedule or cancel the appointment from the email received if the appointment was booked through email. 

You are informed via your business calendar (Google Calendar, Outlook Calendar or the Office 365 calendar) as soon as the appointment is rescheduled or canceled.