try this
"select * from call_Details
where enquirydate between '"+DateTime.Parse( txt_datefrom.Text) + "' and '"
+DateTime.Parse( txt_dateto.Text) +"'";
But this will lead to sql injection.so use sql parameters always
it should be
SqlCommand command = new SqlCommand("select * from call_Details
where enquirydate between @startdate and @enddate", connection)) ;
command.Parameters.Add(new SqlParameter("@startdate" , DateTime.Parse(text_datefrom.Text)));
command.Parameters.Add(new SqlParameter("@enddate" , DateTime.Parse(text_dateto.Text)));