Our phone system is designed to Flip calls to any number in the Call Handling and Forwarding options when the * key is pressed.
You can disable the Call Flip by logging into your extension, selecting the Settings tab, selecting Call Handling & Forwarding, then selecting Call Flip from the bottom of the page. From here move your cell phone number down into the "Unsused" slot. This should resolve your issue.
Aust, did this work for you? I always thought you could not dial * as a leading digit with conference services or auto attendant/IVR systems, etc with RingCentral because it reserved for various features (including call flip)