Hot Desk Phones Not Working

The hot desk phones in two different offices - separate networks with different ISPs - stopped working today. When you tap Login on the Polycom, it says "Service unavailable". Phones programmed with an extension appear to be working. RingCentral support has not been helpful. In researching on my own, I found that sip.ringcentral.com is dropping a lot of pings, which I'm speculating may be related. I pinged this address from three different networks with the same result. Is there another possible cause? We have not made updates to any of our networks or phones in the past several months. Thank you.