Before having to uninstall and-re-install the iPhone app (some time ago) I was able to tell if an incoming call was coming from RingCentral or my own iPhone cell number. Now I can't tell ad can't find the correct setting to change.
What are the settings you currently have for Call Handling & Forwarding? Could you check to make sure your Call Handling & Forwarding is set up so that the first step is "My Desktop App & Smart Phone". If your cell phone number is listed in the settings then the call will connect through your mobile carrier as opposed to the RingCentral app.
If you have the Desktop & Smart Phone app enabled, then the next thing I recommend doing is uninstalling and reinstalling the app again, or updating the version of iOS if there is one available. Let us know how it goes.