Fax Out API - Unable to Send Faxes
Faxes sent via API fail when the API code contains incorrect credentials or incorrect values. To ensure that your API code is correct, you can open the RingCentral Developer Guide for detailed, step-by-step instructions for setting up the Fax API. Look for the Messages section on the Menu, and then click Faxes.
You will find useful references and detailed information to help you get started working in the RingCentral Connect Platform and create your own RingCentral powered app on the RingCentral Developers page.
You can also check out Send Fax Web API (FaxOut API).
Fax Troubleshooting Checklist and Frequently Asked Questions