Nokia 6310 car kit question

How does the Nokia 6310 send the signal to the car kit that it doesn't
turn off the speaker?

Pin 3 = Microphone, 4 = Ground, 5 = Speaker but there must be something
else that avoids the speaker to be turned off. Any idea?
Eventually Pin 9?
(see http://pinouts.ru/CellularPhones-Nok...0_pinout.shtml
for pinout).