demarches-normaliennes/app/models/champs/phone_champ.rb

class Champs::PhoneChamp < Champs::TextChamp
  validates :value,
    phone: {
      possible: true,
      allow_blank: true,
      message: I18n.t(:not_a_phone, scope: 'activerecord.errors.messages')
    },
    if: -> { !Phonelib.valid_for_countries?(value, TypesDeChamp::PhoneTypeDeChamp::DEFAULT_COUNTRY_CODES) && validate_champ_value_or_prefill? }
end