Is there an iPhone texting app that lets you keep your phone number?

I'm on AT&T and it costs $20 a month to send SMS… Is there any app that lets you use your actual phone number without it charging you on your phone bill?

No but there's an app where you can text using Wi-Fi! Textfree or TextPlus, both are good apps.

Watsap, but you can't just send a message to someone's phone number they must have the app, or use the imessage app?