That's false. You can set your phone number discovery to None and even your contacts cannot find you unless you explicitly message them or share your username
Getting a new phone number isn't expensive, but it's infinitely more expensive than zero. And if a service is willing to block the phone companies that offer the cheapest new numbers, the price rises again.
You can buy them in bulk on shady sites for as little as 5c per pop.
Most abuse happens on an industrial scale, and it's trivially easy and practically free to bypass this kind of "security" feature.
Thus no spam on signal.