9to5Google claims to have learned that Google has a new Call Screen-like function for the Assistant in the works. It may become known as "Hold my Phone", and will enable a device's AI to handle a phonecall that has gone on hold by itself. This, apparently, will involve alerting the user when a respondent has come back on to the line.
https://www.notebookcheck.net/Pixel-users-may-soon-be-able-to-get-the-Google-Assistant-to-go-on-hold-for-them.434654.0.html