From Longman Dictionary of Contemporary Englishlisten in phrasal verb1 LISTENto listen to a broadcast on the radio to I must remember to listen in to the news. → tune in2 LISTENto listen to someone’s conversation when they do not want you to on It sounded like someone was listening in on us. → listen→ See Verb table
Examples from the Corpus
listen in• Whenever her boss had one of his "private meetings", she always used to listen in.• We tried to listen in on their conversation, but they were talking too quietly.• They used to have hours of fun listening in on what people were doing in their hotel rooms.listen to• We organise to meet later and listen in to Radio 5.• Since then, police have been listening in to the frequency used by the gang.• Evidently Donald was listening in to what was being said.listen on• Mr Kelbie listened in on an extension.• She did not listen in on his phone calls or search his backpack.• At all times it was possible to listen in on interviews and to monitor the interviewer's work on a control screen.• Bush custom forbade anyone listening in on one of these conversations or interfering with the servants' duties.• The FBI had been listening in on their conversations for months. 