JSFiddle - React, Tailwind, and code Playground

HTML

<input id="telnr">
    <button id="validate">Validate</button>

JavaScript

var button = document.getElementById("validate");
var input = document.getElementById("telnr");

button.addEventListener("click", function(){
    var text = input.value;
    var totalNrOfDigits = 0;
    for(var i = 0; i < text.length; i++){
        if(!isNaN(text[i])){
            totalNrOfDigits++;
        }
    }
    if(totalNrOfDigits < 10){
        alert("Invalid input");
    }
});