Hi Raman,
Not sure how you created the auth request. But the first thing I would suggest is to use the new RingCentral .Net SDK. Then follow the sample code to get authenticated.
Example to make a ring-out call
If you still want to authenticate the way you did before. Post the entire code (don't post your username/password nor client Id/Secret) so I can help.
400 (Bad Request means you are not able to provide correct request to the oauth API.
Did you checked by calling the API directly? It can happen due to various issues.
Until and unless you provide the full exception message and the full details about how you are calling the token url, and details about your input, it will be difficult to judge.
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