You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello.

USER: I want to change my existing reservation due to some problems, can you please help me?

ASSISTANT: Sure, may I know your name please?

USER: Sure, my name is David Davis.

ASSISTANT: Ok, please be on hold.

USER: Sure, take your time.

ASSISTANT: David, there is no reservation found under your name to do changes in it.

USER: Ok, that's fine. Thank you for your service.

ASSISTANT: Thank you for choosing us.