Роман К

SELECT 
  companyName, 
  PriceAvg 
FROM 
  (
    SELECT 
      `companyId`, 
      AVG(`price`) AS PriceAvg 
    FROM 
      phone 
    GROUP BY 
      `companyId`
  ) AS reporttable 
  INNER JOIN company ON reporttable.companyId = company.companyId
  ORDER BY PriceAvg DESC LIMIT 1;