AJAX :: TextBox Control Value Resets To Initial Value, Using WebMethod?

I need to assign a textbox value selected from a modal dialog. The modal dialog displays a list of addresses. I select a particular locationID from the addresslist launched.

I launch the modal dialog from a Link button.

There are Number and street ,city,postocde fields on the .aspx form. I should extract the Number and steet field from the database for which I use the Webmethod. I set the EnablePageMethods value to true of the Scriptmanager control.

I see that the value gets assigned , but resets to the initial value of the NumberandStreet field before the modal dialog is launched.

I am not able to trace as to what could lead to this. Please find the code attached.

<asp:LinkButton ID="LinkButtonPickUpAddress" runat="server" OnClientClick="OpenAddressList()">Select</asp:LinkButton>
<script type ="text/javascript" language="javascript" >
var locationID;
function OpenAddressList() {
var NumberAndStreet;
locationID = window.showModalDialog('AddressList.aspx', null, 'scroll:no;dialogHeight:330px;dialogWidth:500px;centre:yes');
PageMethods.GetNumberAndStreet(locationID, SetNumberAndStreet);
}
function SetNumberAndStreet(ResultNumberAndStreet) {
document.getElementById('<%=TextBoxNumberAndStreet.ClientID %>').value = ResultNumberAndStreet;
}

In the .vb form i have the web method declared this way

<System.Web.Services.WebMethod()> _
Public Shared Function GetNumberAndStreet(ByVal locationID As Integer) As String
Try
Dim theAddress As New IDAL.Address
theAddress = Utilities.m_objDAL.GetAddress(locationID)
Return theAddress.NumberAndStreet
Catch ex As Exception
Return Nothing
End Try
End Function

Note : The above code is placed in the content place holder of a content page