Listening in on a call

Hi everyone, I need to be able to allow authorized users to listen into active calls without the participants of the call knowing someone is listening in to the call. Does anyone know if this is a possibility with Janus? This is only for audio calls, not for video calls.

For the videoroom plugin, you could just join with a new participant without never publishing any feed. Janus by default will notify only active publishers.
The silent participant can then subscribe to active feeds.

Is it possible to use multiple plugins? I am already using the SIP plugin to connect to 3CX to create a IP phone.

Yes, you can reuse the same session with Janus but you would need a new handle and new peer connection.
You could also achieve the same result with 3CX and a muted participant (if the server supports bridge mode).