[tsigel]

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.7/jquery.min.js"></script>

<input type="text" id="phone">

<script>
$('#phone').blur(function() {
  var text = $(this).val();
  if (text[0] != '0') {
    alert("Начни вводить с 0")
  }
})
</script>