Comcast provides an excellent robot call blocking service, Nomorobo. Almost all of the robot calls never ring my home phone anymore. Answering the phone when it does ring usually is a call I want to answer. This service is a blessing and kudos to Comcast for supplying it. I highly recommend it.
However there is one flaw, it blocks all calls from Comcast. A call made during a chat from the representative was also blocked. I had to disable it for them to get through.