Call screening can be setup on the phone or on Xfinity Connect
Here's the link to the features quick guide, basically you can enter the number by dialing *60 or enter it on the Xfinity Connect under preferences / call screening.
How would someone go about making a suggestion to improve this feature? As an example, why not allow users to add phone numbers to a list and when those numbers call, don't allow the call to go through.
It is similiar to what Google Voice does. I've used it and it works pretty well.