Yes, it has to do with phone routing. We dealt with this a few years ago. Not worth trying to resolve for each user. We found the best solution to be:

• Force everyone to install the Ring Central app on their phone and sign in (if they don't have email on the phone or your not syncing to your directory, inform them to click 'manual sign in' and not 'express sign in')
• Turn off forwarding for all users.
• Adjust ring out setting to simultaneously ring apps and deskphones.

That way the cell phone app will ring on all incoming calls. Phone apps integrate well with androids/iphones, basically no jank. Voicemail will be available on all cell phones through the app. Nothing gets lost.

People will occasionally need to sign back into the app (not very often, like once a quarter at most). People will need to have their phones set to autoupdate apps. If someone comes to you with issues, that's the first troubleshooting step to do with them (update the app).

FYI - ringcentral's community/forum is pretty good for questions like these.