function attendee_export_add_phone_split_name( $items ) {
    $count = 0;

    foreach ( $items as &$attendee_record ) {
        // Add the header columns
        if ( 1 === ++$count ) {
            $attendee_record[] = 'First Name';
            $attendee_record[] = 'Last Name';
            $attendee_record[] = 'Phone';
        }
        // Populate the new column in each subsequent row
        else {
            // order id is in the 6th index
            $order = wc_get_order( (int) $attendee_record[6] );
            $attendee_record[] = $order->billing_first_name;
            $attendee_record[] = $order->billing_last_name;
            $attendee_record[] = $order->billing_phone;
        }
    }

    return $items;
}

add_filter( 'tribe_events_tickets_attendees_csv_items', 'attendee_export_add_phone_split_name' );