I am not sure what you want to integrate with mobile devices (let me know more detailed then I can assist you better). But for implementing mobile apps, you can follow this tutorial to create an app for iOS. Using the Swift SDK, you can call any RingCentral APIs as you can do in other programming languages.

RingCentral provides functionality mostly through APIs. Each of their functionality can be performed with API which is actually REST API with a particular format.

Mobile apps can be integrated easily with REST APIs as the mobile apps integrate and perform wells with an API kind of implementation in any platform.

I don't see any issues and you can follow the general guidelines that is mostly practiced everywhere for all other mobile application integration.