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<asp:BoundField DataField="DOB" HeaderText="Date Of Birth" HtmlEncode="False" DataFormatString="{0:d}" />

protected void OnRowDataBound(object sender, EventArgs e)
{
     GridViewRowEventArgs ea = e as GridViewRowEventArgs;
     if (ea.Row.RowType == DataControlRowType.DataRow)
     {
         DataRowView drv = ea.Row.DataItem as DataRowView;
         Object ob = drv["Phone"];
         if (!Convert.IsDBNull(ob))
         {
             Int64 iParsedValue = 0;
             if (Int64.TryParse(ob.ToString(), out iParsedValue))
             {

                 TableCell cell = ea.Row.Cells[4];
                 cell.Text = String.Format(System.Globalization.CultureInfo.CurrentCulture,
                    "{0:(###) ###-####}", new object[] { iParsedValue });
             }
         }
     }
}