https://www.donotcall.gov/US Only.
If its telemarketers, they will have to remove your number from their list within 30 days of subscribing to this service.
Automated systems amaze me. Predictive dialing may be causing the system to call you more than once.
Some people play the call tone that tells the dialer that the number no longer exists.
If you have caller ID, and if you have the number, consider blocking it. That, or dial a certain number (I cant remember, I think its *69, another forum member may verify this, it will tell you the last caller) to find out what the number is.