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Thanks for the additional clarification Ralph. Are you only receiving the 503 error in these NoCall scenarios?

Benjamin,

I work with Dave, we got this error when making a request to get active call information after a NoCall (/restapi/v1.0/account/~/extension/<extension>/active-calls?page=1&perPage=1) notification so that we could get the recording URI (contentUri). This call was made via the Platform object's Get method in RingCentral's SDK.

Ralph

This might be only tribal knowledge (and this question is becoming the reference in the interim).

I will submit a ticket to our documentation team to help improve.

Thank you for the response.  It appears that you are correct.  After an overload, the queued messages are delivered.  We did not have a clear understanding of this process. 
If you know where this is discussed in the Ring Central documentation, I would appreciate a link or reference to the resource.  

AFAIK, when the system reports an overload (at least for Subscriptions), once the transitive error is resolved and normal operation begins again, the queued messages should be transmitted to your application. Was that not the case here?

Yes the message came from the API while running PubNub subscriptions.  This occurred at approximately 1:30 AM and was transitive.  Normal operations began again after approx. a minute.

Hey Dave,

This looks like it came from our API and not PubNub.

Could you tell me more about when this happened, the request you had submitted, and what your use-case is please? I don't wish to make any assumptions and I want to help you get accurate answers.