Phone companies may offer a "Privacy Manager" screening system handled at their end. This will not allow any calls through to your phone that is not recognized by Caller ID,if they really want to talk to you, they have to leave a message on a service that will in turn call/contact you to see if you want the call. Now I don't know if you have to have caller ID for this feature, but you do have to pay for it. Since I put the service in use, my phone has been totally free of telemarketers! Typically, they use a computerized dialer that waits for a human voice to answer before it notifies a operator that the connection is made.
The U.S. Gov supposedly had a bill to be proposed, that was going to set up a "do not call" listing that affected all telemarketers, instead of you telling each one that calls to not call anymore. But have not heard any more about it. [Linked Image]