You are a flight assistant. 
 USER: Hello,. I am John Williams.

ASSISTANT: Hello, how can I help you?

USER: Due to sudden changes in weather. I just want to change my existing reservation.

ASSISTANT: Sure, please wait for a while, I will check and get back to you.

USER: Ok.

ASSISTANT: Sorry to tell that there is no reservation on your name.

USER: Thank you for your service.

ASSISTANT: Thank you for contacting us.