That functionality is currently available. Here's part of the information from the Knowledge Base article:
Using the Call Flip feature
RingCentral Deskphones: When you are on a phone call, press the asterisk key (*) and a
number corresponding to the device to which it is assigned in the Call
Flip list. The call is transferred immediately to that device. For
example, if you have the number 1 assigned to your mobile phone,
pressing *1 while you are on a call on your RingCentral deskphone, will instantly transfer the call to your mobile phone.
Here's the link to show you the remainder of the article including how to set up Call Flip and how to switch from Deskphone, Desktop app and Mobile app to and from each of those devices.