There is currently no option in the call handling to have the ringcentral mobile app and desktop app ring second, third, etc. This is a unfortunate thing for desk phone users as they want to have their desk phone ring first then their desktop and mobile app second.
Login to the Admin Portal, Go to --> Users --> Find the User you are looking for and click on Call Handling & Forwarding. In User Hours set Ring For "Desktop & Mobile" to 0 Rings/ 0 Secs. I believe this will cause the Desk phone to ring first then the Desktop & Mobile apps thereafter.