Is the wife's listening to her mother-in-law's phone conversations with her daughter, with the aim of finding out whether her mother-in-law speaks ill of her, considered forbidden espionage?

It is not permissible to eavesdrop on people's calls or spy on them without their knowledge and consent. For whoever listens to people's conversations while they are averse to it, molten lead will be poured into his ears on the Day of Resurrection. Therefore, it is not permissible for a woman to eavesdrop on her mother-in-law's calls or spy on her, except in specific cases to prevent an evil when suspicion arises.