You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello.

USER: I am Thomas Brown.

ASSISTANT: How may I help you?

USER: Owing to an emergency, I just want to change my existing reservation. Can you please help me to change it?

ASSISTANT: Sure, please wait for a while.

USER: Sure.

ASSISTANT: Sorry, no reservation is available by your name.

USER: Ok, thanks for your help.

ASSISTANT: Thank you for choosing us.