Call Recording feature is not turned on by default in Sandbox profile.
RingCentral's softphone or desktop app can be used to test that.
The RingCentral mobile app as I can see does not support sandbox mode. But still you can use the RingCentral soft phone and switch the soft phone to sandbox mode before login with your sandbox credentials.
On Windows: Ctrl + F2
On Mac OS: Fn + Command + F2
If you wish to create call recordings programmatically, there is an GitHub app that will help you:
https://github.com/anilkumarbp/Sample-Demo-to-Download-Call-Recordings
It takes some time to finish and make the recording available around 30 sec after the call.
You can also refer answer for your question
How to make RingCentral sandbox calls with recording