unwanted phone calls

We have been getting ~6 unwanted calls per day on our "land line". Our anti-spam settings are full on, but the system doesn't work. The calls come from a few different organizations but are from random caller ID's-- never the same twice.

There used to be a challenge option that asked the caller to enter a code before ringing. That would work, but doesn't seem to be available.

BTW- your chat agents don't have any idea how to deal with this problem. Wasted 45 minutes trying to explain.