first you need to get subprocess instances (assuming these are started via call activity) using this method of audit service. Next you can find all tasks for given process instance id using this method of task service. It will return task ids of all tasks so you can then load each task details on separate call. Alternatively you can use this method of task service to load tasks for given status and process instance id which will load task summary so no need to use another call to get task details.
All is clear now.