The RingCentral app allows you to make and receive multiple calls as well as view and manage all the ongoing active calls and handle each of them as needed. You can make a new outbound call or receive an incoming call without ending the call you're already on. This enables you to handle multiple calls that go on simultaneously in the RingCentral app.
You can make and receive up to nine concurrent calls. If you are already handling nine calls, more incoming calls will be routed to other endpoints. For example, that call might route to the RingCentral mobile app. If you try to make more outbound calls, the app will pop-up a message that you've reached the maximum calls limit.