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Here is the way to get access token from refresh token:

https://platform.devtest.ringcentral.com/restapi/oauth/token

username=<your user name>&password=<your password>&extension=<your extension>&grant_type=refresh_token&refresh_token=<your refresh token generated>

Now it will generate new access token from refresh token

As you said you guessed that the access token is expired. In that case, use the refresh token to ask for a new access token. Post this body to the "auth" endpoint

{
  'grant_type' : 'refresh_token',
  'refresh_token' : "your refresh token"
}