Go ahead and give your number away to every website that asks you for it. I don't care what you choose to do.
I don't see this enterprise as anything but malicious if it requires a cell phone just to sign up to see what it can do.
Everything we do is tied to our cell phones now. Most banks offer MFA but not app-based or hardware-based MFA. If / when you lose your cell number, you're screwed - which has happened to a number of people I know. People are dumb and willing to do anything for a thrill without considering the repercussions. If it's not yet an issue, it will be in the very near future.
I hope people seriously take a moment to pause and wonder why this service requires a cell phone number, what they're doing with that number, and how secure the database is that maintains your personal information.
This may be a good moment to remind people; if you're not intending to open a bank account, apply for a credit card, or loan / mortgage soon, you should contact all three credit bureaus to proactively free your accounts so no one can open accounts without your knowledge.