listen in on something/someone
phrasal verb with listen verb [ I ] uk/ˈlɪs.ən/ us/ˈlɪs.ən/
C2 偷听，窃听
If you listen in on a conversation or on someone talking, you listen, especially secretly, without saying anything.
I wish Dad would stop listening in on my phone conversations. 我希望爸爸不再偷听我的电话。