Post form to other page

Hi,
I want to be able to post my form data to another page based on the result of a validation done on server (and not via javascript). 
As an example, let's say the user captures a customer number and then captures the phone number of the customer on page A. When the user clicks on the submit button, the page posts back. Then, in the button click event, I need to check in the database if the customer number exists. If it exists, I need to post the data to page B so that user can input the next information. If it does not, I need to stay on the first page and show an error message telling the user that the customer does not exists.
I do not want to use Response.Redirect cause that would mean that page B would lose the form data coming from page A. I know about crosspage postbacks but that just turn the problem the other way around (if I post directly to page B, how do I get back to page A with the form data and show the error message?)...
I know abou ...