BrPhoneValidator. Validator para Rails (testado em 3.2) para validação de telefones brasileiros. Válido em 1º de setembro de 2013, em constante alteração já que celulares estão gradualmente adotando o 9º dígito.

br_phone_validator.rb

class BrPhoneValidator < ActiveModel::EachValidator
  # first 2 digits represent dialing code
  #   dialing codes in Brazil don't start with 0
  #
  # the following represent the phone
  #   must have 8 digits
  #   first digit must start with 2, 3, 4, 5 or 6
  #
  LANDLINE_REGEXP = /\A[1-9]\d[2-6]\d{7}\z/

  # first 2 digits represent dialing code
  #   dialing codes in Brazil don't start with 0
  #
  # the following represent the phone
  #   must have 8 digits
  #     might have 9 if dialing code is 11, 12, 13, 14, 15, 16, 17, 18 or 19
  #   first digit must start with 7, 8 or 9
  #     might start with 5 as well if dialing code is 11
  # 
  MOBILE_REGEXP = /\A(11(5|[7-9])\d{7,8}|1[2-9][7-9]\d{7,8}|[2-9]\d[7-9]\d{7})\z/

  def validate_each(record, attr_name, value)
  regexp = options[:mobile] ? MOBILE_REGEXP : LANDLINE_REGEXP
  message = options[:mobile] ?
            :invalid_br_mobile_phone :
            :invalid_br_phone

    unless value =~ regexp
      record.errors.add attr_name, options[:message] || message
    end
  end
end

pt-BR.yml

pt-BR:
  errors:
    messages:
      invalid_br_phone: "não está no formato de telefone fixo"
      invalid_br_mobile_phone: "não está no formato de telefone celular"