How to test sandbox in SMS API?

Check out the quick start, choose a language and try it.

https://developers.ringcentral.com/guide/messaging/quick-start

Hi @Jiangning Xing you can also try our API Reference website that has a build in API explorer and you can pick any SMS API you like and after logging in can try it out with your app using sandbox enviornment. See https://developers.ringcentral.com/api-reference/SMS/createSMSMessage -> click on "Sign in to Try out" button and fill in the required fields before making the HTTP POST request.