function isNumeric(sText){
       var validChars = "0123456789";
       var isNumber=true;
       var Char;
       if (sText != null && sText != ""){
           for (i = 0; i < sText.length && isNumber == true; i++) 
           { 
               Char = sText.charAt(i); 
               if (validChars.indexOf(Char) == -1) 
               {
                  isNumber = false;
               }
           }
       }else{
        return false;
       }
       return isNumber;
    }
    
    function showphonepriv(){
                    
      var areacode = document.getElementById('alt_areacode').value;
      var prefix = document.getElementById('alt_prefix').value;
      var suffix = document.getElementById('alt_suffix').value;
 
           defaultDisplay='block';
                                                       
      if (isNumeric(areacode) && isNumeric(prefix) && isNumeric(suffix) && areacode.length == 3 && prefix.length == 3 && suffix.length == 4){
         document.getElementById('phonepriv').style.display = defaultDisplay;
         document.getElementById('survey4').checked = true;
      }
    }