Got two of these fake-IRS scam voicemails within 2 minutes of each other, from two completely different numbers. They've long moved on from landlines to cell phones.
Sigh.
No use trying to stop this from happening--all we can do is erase the voicemails after determining that they are garbage calls (which usually takes less than two seconds).
There is plenty you can do. If they are asking you to call back on the same number, then it's likely not spoofed and this one is an AT&T mobile account. You can either use that information to sue them or have the number shut down.
You should also report all calls to:
https://www.treasury.gov/tigta/contact_report_scam.shtml
Keep very detailed records of each call. Record them if legal in your area, etc. If you do sue them, you can be awarded cash for each call.
Android, Apple and Windows phones all have built in call blocking. You can also download apps that block calls based on known bad numbers. You could also go with a more restrictive approach and make your default ringer silent and only change it for known good numbers.
For landlines you can purchase call blocking devices and phones or abandon traditional service for more capable voip services. My number is well publicized on many organizations contact lists and other websites, so I used to get 15+ spam calls every day. Since I moved to viop.ms and started taking control of my incoming calls, I only get a few per week. I could reduce that further with more restrictive policy's, but I like the balance I have so far.