Example

gistfile1.rb

class User < ActiveRecord::Base
    validates :phone_number, presence: true, length: { is: 11 }, if: :gsm_number_empty?,   unless: :password_or_email_changed?
    validates :gsm_number,   presence: true, length: { is: 11 }, if: :phone_number_empty?, unless: :password_or_email_changed?
private
    def password_or_email_changed?
       encrypted_password_changed? || email_changed? 
    end
    
    def phone_number_empty?
      !phone_number.present?
    end

    def gsm_number_empty?
      !gsm_number.present?
    end
end