What is the sandbox phone number?

Nothing is wrong with your phone number. I see you have 2 apps in sandbox environment. Both apps have the RingOut and SMS app scope and you should be able to call SMS API.

Show me the code how you call the API and the entire error. Make sure that you login your app with the user in your sandbox account.

Your last reply includes a long unclean response and the service blocked it. But I had a look at the first line as saw that it failed at the code where you called the /phone-number endpoint to check the user's phone number and the number feature. This API require the "ReadAccounts" app scope and it is missing from your app settings.

Login the RingCentral developer portal, open your app settings and add the missing app scope and test again.