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The parameter for refresh_token is almost same as grant_type password with minor change.

url will be same: https://platform.devtest.ringcentral.com/restapi/oauth/token

Headers:

"Accept":"application/json“
"Content-Type":"application/x-www-form-urlencoded“
"Authorization",:"Basic <ClientID:ClientSecret in base 64> 

Body:

In body, we need to pass the parameters in following way :

username=<account phone number>&password=<account password>&extension=<your extension>&grant_type=refresh_token&refresh_token=<the refresh token generated>

Thanks for share.This link may everyone know.It is better if you share the parameter,you passed in api to get the refresh token.

Our APIs use Oauth 2.0 Protocol for authentication and authorization. Once the app is authenticated our servers return back the access token and refresh token which are valid for 1 hour and 1 week respectively. You are able to cache the tokens and use them to request new access/refresh tokens after they are expire. We provide different authorization flows for a client application :

• Authorization Code Flow ( recommended for Client/Browser Applications )
• Resource Owner Password Credentials Flow (ROPC)
• Refresh Token Flow

More information on Authorization Flows could be found here : https://developer.ringcentral.com/api-docs/latest/index.html#!#AuthorizationFlows.html