ScriptManager not calling Javascript function

Question

I am trying to call a JavaScript function from codebehind which will show a modal. The JavaScript function is not getting called when using ScriptManager. Please help.

CodeBehind:

using System;
using System.Web.UI;

namespace test.web.controls
{
    public partial class request_callback : System.Web.UI.UserControl
    {       
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        public void open_callback_model()
        {
            ScriptManager.RegisterStartupScript(this.Page, this.Page.GetType(), 
             "open_callback_modal", "javascript:open_callback_modal();", true);
        }
    }
}

JS:

<telerik:RadCodeBlock ID="rcb" runat="server">
<script type="text/javascript">  
    function open_callback_modal() {
         alert('You clicked the button!')
         $get("btn_open_callback").click();
    }
</script>
</telerik:RadCodeBlock>  

Calling open_callback_model() from a different control.

protected void btn_open_request_callback_Click(object sender, EventArgs e)
{
    request_callback callback = new request_callback();
    callback.open_callback_model();
}