we have try to get auth token from RingCentral auth token(/restapi/oauth/token) by using CURL call but we have get
curl -X POST "https://platform.devtest.ringcentral.com/restapi/oauth/token" -H "Accept: application/json" -H "Content-Type: application/x-www-form-urlencoded" -u "clineId:clientpassword" -d "username=username&password=password&extension=101&grant_type=password"<br>Make sure the application you created supports Password flow since your (grant_type=password) .
Please note here, application created with different platform type will have different grant_type.
for more info, here is wonderful way it has answered with animated Gif image that will easily help you to understand the process:
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