  Mass robocalls affect millions of people on a daily basis. Unfortunately,
most current defenses against robocalls rely on phone blocklists and are
ineffective against caller ID spoofing. To enable the detection of spoofed
robocalls, we propose a {\em virtual assistant} application that could be
integrated on smartphones to automatically vet incoming calls. Similar to a
human assistant, the virtual assistant can pick up an incoming call and screen
it without user interruption to determine if the call is unwanted. Via a user
study, we show that our virtual assistant is able to preserve the user
experience of a typical phone call. At the same time, we show that our system
can detect mass robocalls without negatively impacting legitimate callers.
