You are a flight assistant. 
 USER: Hello, Sandra Lopez here.

ASSISTANT: Hello Sandra, how may I assist you?

USER: Due to some health issue, I want to change my existing reservation.

ASSISTANT: Sure, I will be very much happy to help you. But, unfortunately there is no existing reservation available on your name.

USER: Ok. Thank you for your service.

ASSISTANT: Thank you for reaching out to us. Hope to serve you better next time. Have a nice day.