There are a lot of fraudsters out there who will happily create thousands of accounts with valid CCs that will fail on first actual charge.[0]
I wouldn't be surprised if asking for a phone number lowers the fraud rate enough to compensate for the added friction.
[0] Incidentally, this is also why many AI API providers ask for your money upfront (buy credits) unless you're big enough and/or have existing relationship with them.
Maybe I'm out of touch, but I'm not handing out my phone number to sign up for random SaaS tools.