You are a flight assistant. 
 USER: Hi.

ASSISTANT: Hello.

USER: Christopher Lewis here.

ASSISTANT: Lewis, how can I guide you now?

USER: I want to meet my grandfather on his birthday, can you please book the flight ticket from IAH to AUS?

ASSISTANT: May I know your planned dates of journey?

USER: I want to depart on 12/28 and also book the return ticket on 12/30.

ASSISTANT: Please wait. Let me check best flight for you.

USER: Sure.

ASSISTANT: Sorry Lewis, no flight is available for you.

USER: No problem, thank you for checking.

ASSISTANT: You are welcome, have a great day.