Why am I getting "Service is overloaded, please retry later" and "only calling available" errors?

I posted about this a while back, but it keeps happening randomly - sometimes when querying the API I'll get back "Service is overloaded, please retry later". At the same time, our in-office phones (based on RingCentral's desktop software) will usually start showing "Only calling available" in the top right corner.

So, what's the deal?

My logs show this happening today, 9/24, and a bunch on 9/14.

We can retry the API calls, sure, but having our actual phone system "go down" around the same time kinda makes me think something is wrong with RC's backend too. Are the two related?