Minha solução do desafio:
const inputText = document.querySelector('input[type=tel]');
const numberList = document.querySelectorAll('input[type=button]');
for (let counter = 0; counter < numberList.length; counter++) {
const number = numberList[counter];
const numberValue = number.value;
number.onclick = function () {
inputText.value = inputText.value + numberValue;
}
}
