RingCentral provides a good introduction to call monitoring in this video.
A time stamp guide to the video:
00:00 Using a desk phone to answer & monitor others' calls
17:00 Using a desktop app to answer & monitor others' calls
31:00 Advanced: Collaboration and messaging
42:15 User settings - Most demonstrated here have to be changed via ticket to the Service Desk
47:00 User settings - Presence & call handling and forwarding for people using a desk phone to monitor others
48:30 User settings - Voicemail settings
51:00 At this point, we recommend you proceed with the knowledge that RingCentral's help articles can be overwhelming as they write instructions for a number of apps and desk phones