Spoofing caller IDs is currently common because the connection is made all from the caller's end. For my proposed enhancement, a spoofed caller ID fails because it will not be able to return the random key to the caller's telco office. If the caller's telco office does not receive the the random key, it will not generate the validation stamp for the caller ID. The onus is really on the caller's telco exchange to make sure when it receives a random key via a returned caller ID that it can connect that number to an outgoing call request and to a customer that can be tracked via a specific account. Notice that this does eliminate caller ID spoofing, but it allows the receiver to know whether the caller can be traced back to a specific person or company. As an end user I would choose to have my telco block any calls that have not received a validation stamp via the handshake.
Caller ID Screener is a great idea for "real" calls, like blocking the salesman of a company you used several years ago, or an old boyfriend/girlfriend. However, the really troubling calls are not "real" because they fake the caller ID. Faked IDs can be pretty much random, preventing the creation of any useful screening list.