Untitled

<a href="javascript:add_phone()">Add a phone…</a><br />

... and below this, in a script element

<script type="text/javascript">
// <![CDATA[

function add_phone {
  var content = '<li>Phone number <input name="phone_numbers[]" type="text" size="20" /></li>';
  new Insertion['Bottom']($('phone_list'), content);
};

// ]]>
</script>