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.
Indeed as VOIP numbers are banned. However, I know a number of people that aren't using DALL-E because they only have VOIP numbers (This is an ever growing reality today). They got to the phone step and were unable to continue (and support never replied to their requests for help).
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.
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.