Listen in

to secretly listen to a conversation, especially a telephone conversation
Jo had picked up the extension phone and was listening in.
I was sure that someone was listening in on our conversation. [often + on]
The Secret Services are able to listen in to conversations on mobile phones by using a special device. [often + to]

to listen to a radio programme
The programme is very popular and thousands of people listen in every week.
I usually listen in to the 9 o'clock news as I'm driving to work. [often + to]