Textbox inside update panel is still refreshing whole page.

<asp:UpdatePanel runat="server" UpdateMode="Conditional" ChildrenAsTriggers="false">
                               <ContentTemplate>
                               <fieldset>
                                <asp:TextBox ID="txtContact" runat="server"  ForeColor="#999999"
                                    value="Contact Number" onfocus="if (this.value == 'Contact Number')  {this.value = '';this.style.color  = '#000000';}"

                                    onblur="if(this.value == '') { this.value='Contact Number';this.style.color  ='#999999';}"
                                    Width="165px" AutoPostBack="True" ontextchanged="txtContact_TextChanged" ></asp:TextBox>
                                <asp:RequiredFieldValidator ID="rContact" runat="server"
                                    ControlToValidate="txtContact" CssClass="failureNotification" ValidationGroup="Contact" ErrorMessage="*"></asp:RequiredFieldValidator>
                                </fieldset>
                                </ContentTemplate>
                                   <triggers>
                                    <asp:PostBackTrigger ControlID="txtContact" ></asp:PostBackTrigger>
                                    </triggers>
                           </asp:UpdatePanel>

protected void txtContact_TextChanged(object sender, EventArgs e)
     {
         try
         {
             if (txtContact.Text != "Contact Number")
             {
                 string x = txtContact.Text;
                 double y = Double.Parse(x);
                 txtContact.Text = String.Format("{0:(###) ###-####}", y);
             }
         }
         catch (Exception ex)
         {
             txtContact.Text = "Contact Number";
         }
     }