Merci beaucoup, c'est exactement ça.

Voici la requête pour la table cdr d'asterisk :
Code:
SET @cpt = 0;
SELECT evttime, @cpt := @cpt + typevt AS totcalls FROM (SELECT calldate AS evttime, 1 AS typevt FROM cdr UNION ALL SELECT adddate(calldate, interval duration second) AS evttime, -1 AS typevt FROM cdr ORDER BY 1 ASC, 2 DESC) allcalls WHERE evttime > '2010-10-01';