How to show a modal before redirecting on next page when clicked on a submit button in rails?

Consider I have two models: Employer and Jobs. Employer has created an account to post a job and filled in his phone number too. And when he fills up a new job and click on post job button(Submit button). I want to check whether the mobile number that he entered is already verified or not and if it isn't I want a modal or a lightbox to popup where I ask him to missed call on a certain number and as soon as he does that page is redirected to jobs index page. And if he wants to cancel the verfication he can do that but the page will be redirected to nowhere, it'll still be on form page.

But I don't know how to do this. Please, help!