The ShowText="true" property is not taken under account when the OutPutType="EmbeddedPNG" is set:
<telerik:RadBarCode runat="server" OutputType="EmbeddedPNG" LineWidth="1" Type="Code128" Text="1234567" ShowText="true"></telerik:RadBarCode>
ADD GS1-128 standard for RadBarcode Until that's available in UI for ASP.NET AJAX, you can use the Kendo Barcode widget: https://dojo.telerik.com/@bratanov/iPOxiniw