Many proposed solutions to the robocall problem involve blocking calls suspected to be coming from robocallers. This introduces the potential for false positives - blocking a call that was not from a robocaller, or blocking a legitimate robocall. My proposal is to route suspected robocalls to voicemail, if available. This lesser punishment mitigates the downsides of false positives, making aggressive robocall identification via crowdsourcing practical.
In the first phase, the feature would be limited to cloud-based voicemail offered by telephone companies. In a later phase, answering machines would detect a "Route to Voicemail" signal, allow quick review of messages suspected of being spam, and support flagging additional messages as robocalls (i.e. those messages the phone company didn’t identify as a robocall).
The new feature is workable, easy to use, and can be rolled out in phases at very low cost.