Miscellany

SELECT t1.*, count(t2.id_call) cnt
  FROM calls t1
  LEFT JOIN calls t2
    ON t1.id_route = t2.id_route AND
       (t1.call_start, t1.id_call, t1.duration) <= (t2.call_start, t2.id_call, t2.duration)
 GROUP BY (t1.id_call, t1.duration)
 HAVING cnt <= 3;