PHP - Worldpay Callbacks

Hi guys,

This is driving me mad, and I'm wondering if it will be resolved when I set the site live.

I have a form with a hidden field containing the callback URL. The form is submitted to Worldpay.

I am creating a new site in Wordpress although the callback file is a normal php file (ie not Wordpress).
However, a successful payment results in staying at the standard Worldpay success pay instead of redirecting to the callback.
This is being hosted on a temporary domain.

I've tried the following combinations:
Form on temp website to callback on current site - works OK.
Form on temp website to callback on temp site - callback doesn't work.
Form on current website to callback on current site - works OK.
Form on current website to callback on temp site - callback doesn't work.

If I take the form and put it on the domain I will be using, the callback seems to operate as I would expect.

The other thing is that currently the site is on a dedicated server, and the temporary site is on a shared server - and will be staying there.

Is this something that sounds like it will be resolved when the new site is hosted on the proper domain?

Thanks!