How to Add dummy calls and call recordings to sandbox for development and testing?

You can download the RC Desktop app (choose Windows or MacOS accordingly). Install the RingCentral softphone and switch it to the "Sandbox mode" (fn + cmd + f2 for Mac and Fn + F2 for Windows). Login the softphone with your sandbox user's login credentials and make phone calls to/from the softphone. You can record your calls too.