I remember doing something similar to get ISP access in the early 90s. Like AOL various ISPs would be bundled into CDs/floppies and offer a free trial. One of them did not validate the card number, so you could just type in 000000000000 or whatever and your free trial was enabled for a month or whatever and then be auto cancelled when they tried to bill for your first month after the trial. In the UK though local cal…
Presumably the activation of service happened locally in the device, or with minimal cross-checking with the backend billing service.
A few months later I tried it on another unit and the number no longer worked for activation :)