<script type="text/javascript">
           var <%=ClientID%>_getNumber = function () {
            return alert( <%=txtCountryCode.ClientID%>.getValue());
        }
    </script>
 
<ext:CompositeField runat="server" ID="eCompositeField">
    <Items>
        <ext:Label ID="PhoneBoxTitle" runat="server" />
         <%-- ------------------------ %>
    </Items>
</ext:CompositeField>

<uc:PhoneTextBox2 runat="server" ID="txtb" />
<ext:Button Text="test" runat="server">
        <Listeners>
            <Click Handler="#{txtb}_getNumber();"></Click>
        </Listeners>
</ext:Button>

<Click Handler="#{txtb}.getNumber();">