[Dilettante_Pro]

<span class="phone_number">+7495333<span class="mask">2211</span><span class="phone_number_active"> Спрятать</span></span>
<script>
var masked = document.querySelector(".mask").innerText;
function closePhone() {
      document.querySelector(".mask").innerText = "XXXX";
      document.querySelector(".phone_number_active").innerText = " Показать";
};
function openPhone() {
      document.querySelector(".mask").innerText = masked;
      document.querySelector(".phone_number_active").innerText = " Спрятать";
};
document.querySelector(".phone_number_active").onclick = function() {
    if(this.innerText == " Спрятать") closePhone();
    else openPhone();

}

</script>