LADYX,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.phone_number_active{
position: relative;
margin-left:40px;
}
.phone_number_active:before {
position: absolute;
content: "XXXX";
left: -80px;
background-color: #FFFFFF;
margin-right:30px;
}
.phone_number_active:active:before {
content: ""
}
</style>
</head>
<body>
<span class="phone_number">+74953332211<span class="phone_number_active"> Показать</span></span>
</body>
</html>