public function buildForm(FormBuilderInterface $builder, array $options)
    {
        //国際電話番号フレフィクスフィールド
        $builder->add('international_phone_number_prefix', ChoiceType::class, [
            'placeholder' => 'common.select__unspecified',
            'choices' => $this->CountryChoices,
            'required' => true,
        ]);

<dl>
                                <dt>
                                    <label class="ec-label required">{{ '国番号'|trans }}</label>
                                    <span class="ec-required">{{ 'common.required'|trans }}</span>
                                </dt>
                                <dd>
                                    <div class="ec-input">
                                        <span class="customer-form customer-international_phone_number_prefix"></span>
                                    </div>
                                </dd>
                            </dl>

<input type="hidden" id="customer-international_phone_number_prefix" class="customer-in" name="customer_international_phone_number_prefix" value="{{ Customer.international_phone_number_prefix }}">
                        <input type="hidden" id="customer-phone_number" class="customer-in" name="customer_phone_number" value="{{ Order.phone_number }}">