Spam calls

There is not much you can do. No mechanical systems to stop phone calls.

Train users to answer Name or Hello, wait not more than 1 second for a response and then hang up.

As John states there's no mechanical system to stop the phone calls.

But there are Apps and potentially Phone provider tools that might help eliminate SOME of the unwanted calls.

https://www.theverge.com/2018/3/6/17071478/spam-calls-how-to-stop-block-robocalls-robots-scam-iphone-android

Only going to get worse.
Predictions are that by 2019 almost half of all phone calls will be spam.
https://www.washingtonpost.com/technology/2018/09/19/nearly-half-cellphone-calls-will-be-scams-by-report-says/?utm_term=.39b4a7e9c849

Thanks all for response.

I forgot to mention this started happening at an increased pace after we switched phone vendors. Both are reputable vendors. We transferred the DIDs so it's not about new numbers or anything like that but I don't think it's a coincidence this started after the switch. Would it be possible one of the vendors sold our numbers to a third party?

Thanks!

Good vendors do not usually sell phone numbers, but less reputable one might

Very highly doubtful that your phone numbers were sold to a 3rd party.  Especially with a respectable reputable business.
There are sooo many spam callers that hit all the time.  A few months ago I was getting hit maybe 3 to 5 times a day with 2 or 3 different Spammers.  And it went on for a long while.  Then now I believe I am at a lull.  So that's good for me.  There's not a whole lot that can be done unfortunately.
I ride it out as it happens.  It seems to happen to me every few months I would get bombarded.  And then a lull.  Now I am on a lull.

My GF however is getting bombarded.  Similar spam and are actually leaving messages now.  But we know them to be spam.  So we just ignore it for now.  Her dad got scammed and they paid some money.  I told them not to, but  her parents got scared and paid anyway.  So sad...

An automated attendant with a 5 - 10 second non-action hangup rule might help

Looks as if they are going to do a variation on SSL though.
All calls routed through switches will need some kind of authentication mechanism in the future.
I saw where they were going to spend a $100 million on the infrastructure in a few years.

I saw where they were going to spend a $100 million on the infrastructure in a few years

Yea... riiiight

They have to do something ,otherwise the system will bog down and all we'll get is your call cannot be completed.

https://www.mercurynews.com/2018/10/15/robocalls-finally-big-phone-carriers-to-take-promising-step-to-reduce-consumer-scourge/