Accessibility and semantic
The button element (mc-phone-number__button) must always have an id, a aria-haspopup, aria-labelledby and the type="button" attribute.
The list element (mc-phone-number__list) must always have a role="list" or role="listbox" attribute.
Each list element must have the role="option"
Add the aria-selected="true" attribute to the selected list element.