The doctor will call you either sometime in the morning (AM Appointment) or sometime in the afternoon (PM appointment). A reminder will be sent to you with an appointment time but this time simply represents whether the callback is AM or PM and is not the time the doctor will call.
If the doctor is unable to contact you the first time they try, they will attempt one more time. If they have failed to make contact on two occasions then they will not try again and you will have to re-book your appointment.