The "regular" methods provided with XP (phone and online) don't work anymore. The phone number is not valid for activation anymore (I tried the German one) and the online activation does not work either (in a quick test within a DMZ other protocols like ICMP or HTTP worked fine when I accessed an internet server).
Calling the current MS activation number for a product still in support (e.g. for activation for Win7) and choosing "Other Products" in the voice menu. This is my preferred one since you don't have to tamper with the system. In my case (Nov 2019) I followed the link to the Microsoft Licensing Activation Centers worldwide telephone numbers.
This link just allows you to manually activate the the installation. So you select activate via phone option, then your XP box activation screen will give you a set of numbers. You enter these numbers into the site and the site gives you the response numbers to enter into the XP installation that you are needing to activate. Then you click on "next" and the activation completes.
A wildly wrong date will break anything that uses ssl/tls, as certificates will be assumed to be either expired (if the date is in the future) or not yet valid (if the date is in the past). This apparently includes windows activation.
I just successfully activated an old copy of Windows XP I had installed using the toll free U.S. phone number listed in the activation dialog (888-571-2048), so it's possible the issue was with the German number and not phone activations in general. 2b1af7f3a8