What is the ruling on secretly recording family phone calls to ward off فساد (mischief/corruption)?

Espionage is forbidden, as Allah Almighty says: "And do not spy." We are only to judge by what is apparent. However, if the guardian of the house sees signs indicating malicious conversations, he is permitted to place a recorder without their knowledge. He must then immediately rebuke the offending party upon learning of the bad conversation and not delay in doing so. As for mere accusations and baseless suspicions, they do not justify espionage.