Listing people with their phone numbers (if any)
What you are describing is known asLEFT OUTER JOIN

SELECT
   id,
   name, 
   group_concat(
      concat(
         number,
         ' (',
         description,
         ')'
      ) ORDER BY description SEPARATOR ','
   ) AS phones
FROM people LEFT OUTER JOIN phones
ON person_id = id
GROUP BY id;


inner join => give me only rows with records in both tables
left outer join => give me all rows of from the left table along with any matching rows in the right..