Looking for WordPress Plugin Developer

Hi, I am creating a WordPress plugin for Twilio. Everything is works just fine. But, the problem is when the form is submitted then I want to show a confirmation message below the form. I used echo "Your message has been sent!" but it's echo a blank page. It is because may be WordPress form action is using a different way and I don't know about it or I might missing something. I just want to echo the message to the current page. And I made it as a WordPres shortcode. So I can use it my website in any places. So, it should echo to the current page! Let me know who can help? I am attaching the plugin here so you can see!