While Integrating Faxing in Salesforce to Ringcentral We are having having 400 Error Bad Request??

If you want a better chance for help here on the developer community, then put a few lines of codes (without any sensitive information such as a real phone number etc.) around the area that causes the problem in your question. Theoretical question requires an entire book on error handling, while some exact code probably requires a few lines of suggested code to fix.