a phone number is a very private thing so your explanation is unwarranted. you can be concerned about giving away your phone number without having concerns for anon
I would guess that for many people their mobile phone number is a identifier more stable and unique than even their full name. In some jurisdictions it's hard to get one without presenting an ID.
except as a spam impedence (not prevension) requiring phone numbers is a legitimate security and privacy risk, especially for people who think it is a stable and unique identifier
