Phone verification raises account creation costs to at least 20-30 cents an account (assuming you ban VOIP and only take first world countries, can be lower otherwise), email verification if you still allow yandex rambler outlook hotmail addresses raises it to a cent at max.
Take my parent's complaint and replace email with phone and the complaint gets worse since the privacy concerns are even greater.
Most email services now demand SMS verification to sign up for them now as well.