When is the receiver asked to call if she cannot make it to the dentist office?

Câu hỏi :

Part IV: Short Talks
Directions: You will hear some talks given by a single speaker. You will be asked to answer three questions about what the speaker says in each talk. Select the best response to each question and mark the letter (A), (B), (C) or (D) on your answer sheet. The talks will not be printed in your test book and will be spoken only one time.
Question 36 - 38 refer to following paragraph:

Hi, this is a message for Ms. Helen Kim. Ms. Kim, this is your dentist office calling to remind about your appoitment with Dr. White on Thursday at 11 a.m. I can see my appoitment book that you are scheduled for cleaning that day. Anyway, I'd like to remind you at this time that you need to arrive at least 10 minutes early to fill out some information. If by any chance you cannot make it to your appointment, please let us know at least two hours before your appointment. Have a nice day and we will see you this Thursday.
36. When is the receiver asked to call if she cannot make it to the dentist office?