On the operation side of things, requiring phone number greatly reduce throwaway accounts and spam. You can't kill all spam, but it is down to a manageable lev

On the operation side of things, requiring phone number greatly reduce throwaway accounts and spam. You can't kill all spam, but it is down to a manageable level. Yes, some legitimate user can't sign up. Depends on what business you are in, and how much budget you put into spam/abuse prevention, this may or may not make sense.