You are a flight assistant. 
 USER: Hello. Pamela Johnson here.

ASSISTANT: Hello, how may I help you?

USER: I would like to prefer a flight with lay-by option for my business trip to Houston. Can you book the flight ticket?

ASSISTANT: Sure, may I know your airport codes?

USER: The airport codes are from DFW to HOU.

ASSISTANT: When do you want to travel?

USER: The travel dates are from Aug 11 to Aug 13.

ASSISTANT: Can I know your other preferences?

USER: That is it.

ASSISTANT: Sorry, there are no flights available on your travelling dates.

USER: Not a problem. Thank you.

ASSISTANT: Thank you for choosing us.