string strUserAgent = Request.UserAgent.ToString().ToLower();
if (strUserAgent != null)
{
    if (Request.Browser.IsMobileDevice == true || strUserAgent.Contains("iphone") || 
        strUserAgent.Contains("blackberry") || strUserAgent.Contains("mobile") || 
        strUserAgent.Contains("windows ce") || strUserAgent.Contains("opera mini") || 
        strUserAgent.Contains("palm"))
  {
        Response.Redirect("DefaultMobile.aspx");
  }