You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, may I know your name please?

USER: My name is Margaret Lopez.

ASSISTANT: How may I be of your assistance today?

USER: I want to book a flight ticket with a price limit of 500 for celebrating my birthday in Phoenix.

ASSISTANT: OK.

USER: My departure day is 07/18, return day is 07/20.

ASSISTANT: Please update me your travel dates, on which you want to travel?

USER: My source and destination airport codes are TX-DFW, AZ-PHX.

ASSISTANT: OK. Please wait a while, I will check and let you know the availability of flights.

USER: Ok.

ASSISTANT: Thanks for waiting, we found a flight in Southwest airline with fare 200. Shall I proceed to booking?

USER: Yes,please.

ASSISTANT: Your booking has been done with flight number 1020.

USER: Thanks for your service.

ASSISTANT: Thanks for contacting us, have a safe journey.