ログ内の各呼び出しについて、そのcall_datetimeでアクティブな呼び出しをカウントし、最大値を見つけます。
SELECT MAX(calls) FROM
(SELECT cl1.call_datetime, COUNT(*) AS calls
FROM call_log cl1
INNER JOIN call_log cl2
ON cl1.call_datetime
BETWEEN cl2.call_datetime AND ADDTIME(cl2.call_datetime, cl2.call_length)
GROUP BY cl1.call_id) AS counts;