JSFiddle - React, Tailwind, and code Playground

HTML

<input id="phonenumber" type="text" name="foo" />
<p id="isNumber">Please enter a valid phone number!</p>

JavaScript

function checkForNumber(number, size) {
    var nbrString = '';
    for (var i = 0; i < number.length; i++) {
        var cc = number.charCodeAt(i);
        if (cc >= 48 && cc < 58) {
            nbrString += number[i];
        }
    }
    return nbrString.length === size;
}

$(document).ready(function() {
    $('#phonenumber').keyup(function() {
        var cfn = checkForNumber(this.value, 10);
        var a = "This phonenumber is valid! :)";
        var b = "This phonenumber is not valid! :(";
        $('#isNumber').text(cfn ? a : b);
    });
});