I am getting an error using the Production credentials for my app; the Sandbox credentials had worked fine and I didn't run into any issues. The credentials I'm using for Production are the ones that match what are provided in the Credentials view when I'm logged into my developer account, including the listed phone number and extension.
This is the error message that I get when using those Production credentials:
{"errorCode"=>"FeatureNotAvailable", "message"=>"Phone number doesn't belong to extension", "errors"=>[{"errorCode"=>"MSG-304", "message"=>"Phone number doesn't belong to extension"}]}
Similar (though different error message) to https://forums.developers.ringcentral.com/questions/1117/send-sms-featurenotavailable-after-moving-app-to-p.html, but wasn't able to follow the link in the answer to that.
