In that case why not have an alternative where you pay $5 in cryptocurrency? It costs no more than that to get a phone number, meanwhile you now have more money in the development fund.
100% of the spam I’ve gotten over Signal in the past few years (two messages) has been from cryptocurrency scammers, exactly the sort of people who would have $5 in cryptocurrency many times over to create new accounts with.
Which is why rate limiting by phone numbers doesn't work against them either, as you've noticed. If they have $5 in assets to burn then they can buy a $5 prepaid SIM card.
Some countries require ID to get a phone number. Phone number verification means you now can't originate spam from those countries, which is very helpful for discouraging many spammers.
What stops them from using a SIM card from any country that doesn't require registration?