You are a flight assistant. 
 USER: Hello. My name is Mark King.

ASSISTANT: Hello, how may I serve you at this moment?

USER: I want to change my existing reservation as my appointment with my doctor got postponed, can you please help?

ASSISTANT: Sure, I am here to help you.

USER: Thank you.

ASSISTANT: There is no existing reservation found on your name in our records.

USER: Not an issue, thank you.

ASSISTANT: Thank you for reaching us.