Dic: listen in (on someone or something)

• The band is rehearsing. Let's go listen in on them. 
• It won't hurt to listen in, will it? 
• Please don't try to listen in on us. This is a private conversation. 
• I am not listening in. I was here first. You are talking too loud.

listen in (on someone or something):

1. to join someone or a group as a listener. 
2. to eavesdrop on someone. 

... http://idioms.thefreedictionary.com/listen+in