ozgdesign,
I think - but you might want to check - that WorldPay do not allow the use of ip addresses for callback.
My post here
http://www.zen-cart.com/forum/showpo...&postcount=285 describes how the module constructs the callback url.
I suggest checking that when moving your domain from the old server to the new one that you have correctly updated the name servers and/or IPSTAG details and that the includes/configure.php and admin/includes.php files on the new server have been updated to reflect the new server details.
As you suggest this sound like a DNS problem. I doubt that WorldPay cache DNS details - but your ISP might.
Alan



