Someone

SELECT
      DATE_FORMAT(MIN(calls.`datetime`), '%Y/%m/01') as `Period`,
      campaigns.alias as `Campaign`,
      COUNT(*) as `Response`

FROM calls

JOIN campaigns ON calls.campaign_id=campaigns.id

GROUP BY DATE_FORMAT(`datetime`, '%Y/%m/01'), calls.campaign_id

ORDER BY DATE_FORMAT(`datetime`, '%Y/%m/01') DESC