LADYX,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.phone_number_active:before {
content: "XXXX"
}
.phone_number_active:active:before {
content: attr(text)
}
</style>
</head>
<body>
<span class="phone_number">+7495333<span class="phone_number_active" text="2211"> Показать</span></span>
</body>
</html>