JSFiddle - React, Tailwind, and code Playground

HTML

Name: <input type="text" name="Name" value="Name" id="fname"> <br>
Email: <input type="text" name="Email" value="Email" id="mail" > <br>
Phone: <input type="text" name="Number" value="Phone Number" id="dvr">
<span id="phoneval"> ?</span>

JavaScript

document.getElementById("dvr").onchange = function() {
     if (this.value.length == 10){
            document.getElementById('phoneval').innerHTML="-- Valid";
        }
        else{
            document.getElementById('phoneval').innerHTML="-- Not Valid";
        }
    };