I use a CRM application called Connectwise. It supports TAPI Sources for dialing. I've installed the RingCentral Desktop app for windows, but it does not seem to install itself as a "TAPI Source" in windows. Is RC supported as a Tapi source?
I also use ConnectWise and although it's true that you can't use the TAPI intergration, you can use the F6 hotkey to dial any number you have highlighted. So personally I just highlight the number on the ticket/contact/company record and press F6 to make RingCentral Desktop dial that number. True, it's a workaround but at least it's something :)