@natanji Oh yes, you are right, it's a virtual number, like this one: https://smsreceivefree.com/info/13433035367/
But how do we know that number isn't used by another person? Oh well, it is used (as I can see on that page), so probably it's helpful for some people.
@jeroenpraat That's the point; it might be used by another person, but each number can be registered for one Signal account as well, and said account can then be locked to disallow any modifications/another user taking over.