Jonathan, see https://developer.ringcentral.com/api-docs/latest/index.html#!#APIRateLimits.html

The fact that you are getting 429 does not cause token invalidation. There are multiple possible reasons why refresh tokens may become invalid:
* Previously completed refreshment transaction (old tokens become invalid)
* End user password change
* Direct revocation via API
* Session invalidation by end-user via Service Web portal
* Expiration
* Reaching the limit of simultaneously active sessions.

Hope it helps. If you provide some reference to your account (client_id or RC account ID or organization name) I can take a look in our logs about your particular case.

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One more reason the token can get invalidate is if someone calling https://platform.devtest.ringcentral.com/restapi/oauth/revoke API and passing the active token to revoke. This will also make a toke invalid

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