Idea

You are not alone!  We are a small business experiencing the same issue and have gotten a similarly nonchalant response from RC (after all, more $$$ for RC).  My feeling is that this may be a widespread issue for RC customers, many of which are too big to notice the nickel-and-dime nature of these charges.  Strongly agree that the IVR needs to be able to disconnect after 1 time through the announcement, not 3.

RC's earnings just beat estimates by 31% - makes you wonder how much of all that new revenue came from tens of thousands of 50-cent ghost calls nobody noticed...

We have experienced the exact same issue.  RingCentral support has "no solution" to the matter.  Fraud department can't help. 
The number of 1-800 calls we started receiving once becoming a RingCentral customer tripled and continues to increase.  I believe RingCentral is complicit in the 1800 robocall scam. 

Has anyone else experiencing this issue seen a drastic decrease in robo 1800 calls starting on around 9/6/19?  

Thousands of RingCentral have been complaining about robocallers, spam calls, telemarketers for YEARS. There are many many forum threads on the issue. I haven't researched it recently, but it was determined that previous RingCentral Terms of Use included the legal ability to sell our numbers to 3rd parties for marketing purposes AKA robocallers, among other organizations.  (At one time, I couldn't believe it, I read the terms and saw it myself.)  RC actually make a profit on selling our numbers... costing us all a lot of money. How/why would they ever give us a good option to stop the junk calls? (In fact, they may be contractually obligated to never give us the option - but all those they sold our numbers to - we don't know, those are private agreements!) It's simply an added cost to their 'reasonable prices'. If you search the forums, surely you'll run across the posts. 

Regretfully, recently as last year, after shopping around and talking to many millennials at different hosted phone providers, who don't understand POTS phone lines are still a thing - with real copper phone lines attached at the wall - we found RC still offers the best solution for us.  It's unfortunate the cost is so very high - when we include the cost to pay folks to answer junk calls throughout the day.  

Not to worry, although this seems to go on  forever, nothing is really forever.  It will change soon, for better or worse.  We are still very thankful for the phone service RC has provided over the years.