WooCommerce - Adding new "My account" registration fields

functions.php

<?php
/**
 * Add new register fields for WooCommerce registration.
 */
function wooc_extra_register_fields() {
    ?>

    <p class="form-row form-row-first">
    <label for="reg_billing_first_name"><?php _e( 'First name', 'woocommerce' ); ?> <span class="required">*</span></label>
    <input type="text" class="input-text" name="billing_first_name" id="reg_billing_first_name" value="<?php if ( ! empty( $_POST['billing_first_name'] ) ) esc_attr_e( $_POST['billing_first_name'] ); ?>" />
    </p>

    <p class="form-row form-row-last">
    <label for="reg_billing_last_name"><?php _e( 'Last name', 'woocommerce' ); ?> <span class="required">*</span></label>
    <input type="text" class="input-text" name="billing_last_name" id="reg_billing_last_name" value="<?php if ( ! empty( $_POST['billing_last_name'] ) ) esc_attr_e( $_POST['billing_last_name'] ); ?>" />
    </p>

    <div class="clear"></div>

    <p class="form-row form-row-wide">
    <label for="reg_billing_phone"><?php _e( 'Phone', 'woocommerce' ); ?> <span class="required">*</span></label>
    <input type="text" class="input-text" name="billing_phone" id="reg_billing_phone" value="<?php if ( ! empty( $_POST['billing_phone'] ) ) esc_attr_e( $_POST['billing_phone'] ); ?>" />
    </p>

    <?php
}

add_action( 'woocommerce_register_form_start', 'wooc_extra_register_fields' );