If the driver is likely to motor on, the call is diverted to voicemail.

ECONOMIST: Computing

The call went straight to voicemail.

ENGADGET: We've got iPhones, what do you want to know?

Sometimes the software mistakenly identifies an answer machine or voicemail, and terminates the call, even though it has been answered by a human being.

BBC: Silent calls: TalkTalk fined ?750,000 by Ofcom