Welcome to the RingCentral Community

"...[L]ogin the sandbox service web and add more users (up to 4 total per sandbox account). Then open different browsers (e.g. one with Chrome and one with Safari) and make phone calls between those users."

Or maybe just provide dummy data in all sandbox accounts so we don't have to endure this nonsense?

That's perfectly acceptable. But it would nice to have demo data to exercise and understand API call functionality.

There are several things you need to do.

If you are in the U.S and don't mind to make call between your own mobile number and your sandbox number, then just login the RC app for sandbox and make phone calls to generate call log data. If it does not allow you to make direct call, you may need to login your sandbox service web and add direct number to the user extension.

If you are not in the U.S or you want to make calls between users in your sandbox, login the sandbox service web and add more users (up to 4 total per sandbox account). Then open different browsers (e.g. one with Chrome and one with Safari) and make phone calls between those users.