[Fixed]-Data attribute for ChoiceField option

If you insist on rendering the field manually, then you also have to determine manually which item is selected, and output the selected attribute on the relevant one.

{% for choice in phone_code_field.field.extra_data %}
    <option {% if phone_code_field.field.value == choice.0.1 %}selected="selected"{% endif %} data-choice-code="{{ choice.0.0 }}" value="{{ choice.0.1 }}">{{ choice.1 }}</option>
{% endfor %}