Well it's a shame they still want my phone number. I don't wanna give some random company my number if they aren't gonna call me.
Specifically, signup process is: email, email-verification, create-password, full-name, phone. Leaving the process to try the login will return you to the request for a phone.
At least in the USA, getting hold of large numbers of phone numbers for free isn't easy.
Even for a one-time-use for verification, last I looked for one-off number verifications, it was $1 per authentication; to be fair, didn’t search too hard.