1) Check if you set them up for your flight. More info in this article. Reminder: You need to manually add a subscriber to each upcoming flight.
2) The number should be entered at the international format (starting from the + sign). Here is the list of codes by the country: https://countrycode.org/
3) Check your provider's settings. In some countries, carriers may block such text messages. In this case, you need to contact your carrier to whitelist our text messages.