I already know the function 'on_sent_ok' to add jQuery actions when the form is submitted with success. However I want to display a modal window alerting the user when there are submitting erros. There's some function like the one above, but to be called on errors?
In version 3.3 new jQuery custom event triggers were introduced:
New: Introduce 5 new jQuery custom event triggers (invalid.wpcf7, spam.wpcf7, mailsent.wpcf7, mailfailed.wpcf7, submit.wpcf7).
See: http://contactform7.com/2012/09/24/contact-form-7-33/#more-2598
You can use invalid.wpcf7
like the example below:
$(".wpcf7").on('invalid.wpcf7',function(e){
// Launch your modal window here
});