How can I shadow calls on RingCentral?

I believe you have to have the Call Center subscription w/Ring Central.  Added costs.

In the Admin Portal, if you're enabled, you should see Groups on the Left under the Phone System tab.  There should be a second menu in there for Call Monitoring.  You'd setup your users to monitor and who can be monitored there.  When they want to monitor the trainee, just dial *80 and then the extension to monitor.  This can be done from a desk phone, soft phone or the mobile app.  The Trainee and the other party do not hear you.  *81 will allow whisper coaching and *82 will allow barging into the call.  
The end users get notification when this feature is turned on or off.  
read more hear.
https://support.ringcentral.com/s/article/8086