Grid view can't show result (using QueryExtender) when chosed an other Grid'view page

My page have a EntityDatasource

<asp:EntityDataSource ID="SearchEntityDataSource" runat="server" 
        ContextTypeName="CallSellSupport.DataAccess.supportCallEntities"   
        EnableFlattening="False"  EntitySetName="Customers" Include="Area" OrderBy="it.CreatedDate DESC" 
        ConnectionString="name=supportCallEntities" DefaultContainerName="supportCallEntities">
    </asp:EntityDataSource>

and QueryExtender

 <asp:QueryExtender ID="SearchQueryExtender1" runat="server"  TargetControlID="SearchEntityDa开发者_JS百科taSource">

           <asp:SearchExpression SearchType="StartsWith" DataFields="Phone">
                <asp:ControlParameter ControlID="txtPhoneNum" />
            </asp:SearchExpression>
             <asp:SearchExpression SearchType="Contains" DataFields="FullName">
                <asp:ControlParameter ControlID="txtCustomerName" />
            </asp:SearchExpression>
    </asp:QueryExtender>

and a GridView (allow paging)

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" 
        DataKeyNames="CustomerID" DataSourceID="SearchEntityDataSource" 
         AllowPaging="True"  AllowSorting="True" CellSpacing="1" 
         PageSize="7"

I entered data into texbox, then I clicked button. Gridview display ok :)

But when I chosed Page 2 (or page 3, or page 4), then I enter data into textbox, and now, gridview doesn't display anything data, although I clicked button (many times...)

I think this problem is post back (because I change Gridview's page), but I can't find out a solution.

Hope somebody may help me...

Thanks a lot.

---

Please don't forget to back to the 1st page

    protected void Button1_Click(object sender, EventArgs e)
    {
        GridView1.PageIndex = 0;
    }