<?php $form = ActiveForm::begin(['id' => 'form-signup', 'options' => ['enctype' => 'multipart/form-data']]);?>
<?= $form->field($model, 'username') ?>
<?= $form->field($model, 'email') ?>
<?= $form->field($model, 'phone') ?>
<?= Html::buttonInput('+Добавить телефон')?>
...
<?php ActiveForm::end(); ?>