자바스크립트 전화번호 유효성검사요

<HTML>
 <HEAD>
  <SCRIPT language="javascript" type="text/javascript">
  //<![CDATA[
    function phoneCheck(obj) {
      var n = obj.value.replace(/\-/g, "");
      var len = n.length;
      var number=n;
      if(len > 3){
        number=n.substring(0, 3)+"-";
        if(len > 3 && len < 7){
          number+=n.substring(3);
        }else if(len > 6 && len < 11){
          number+=n.substring(3, 6)+"-"+n.substring(6);
        }else if(len == 11){
          number+=n.substring(3, 7)+"-"+n.substring(7);
        }
      }
    
      obj.value = number;
    }
  //]]>
  </SCRIPT>
 </HEAD>
 <BODY>
  <INPUT TYPE="text" onkeyup="phoneCheck(this)" maxlength="13">
 </BODY>
</HTML>