I never even knew about this #77 feature!
This actually happened to someone's daughter. 
Lauren was 19 yrs old and in college.

This story takes place over the Christmas/New Year's
holiday break.  It was the Saturday before New Year's
 and it was about 1 PM in the afternoon, and Lauren was
 driving to visit a friend.

An UNMARKED police car pulled up behind her and put
 his lights on.  Lauren's parents have 4 children (high
 school and college age) and have always told them never
 to pull over for an unmarked car on the side of the
road, but rather wait until they get to a gas station, etc.
So Lauren had actually listened to her parents advice,
 and promptly called #77 on her cell phone to tell the 
police dispatcher that she would not pull over right 
away. She proceeded to tell the dispatcher that there
 was an unmarked police car with a flashing red light on
 his rooftop behind her.  The dispatcher checked to see
 if there was a police car where she was and there
 wasn't and he told her to keep driving, remain calm and
 that he had back-up already on the way. Ten minutes
 later 4 cop cars surrounded her and the unmarked car
 behind her. 
One policeman went to her side and the others
 surrounded the car behind. They pulled the guy from 
the car and tackled him to the ground..........the man was 
a convicted rapist and wanted for other crimes. I never 
knew that bit of advice, but especially for a woman 
alone in a car, you should not pull over for an unmarked 
car. Apparently police have to respect your right to 
keep going to a "safe" place. You obviously need to
make some signals that you acknowledge them (i.e. put on
 your hazard lights) or call #77 like Lauren did.

Too bad the cell phone companies don't give you this
 little bit of wonderful information. So now it's your 
turn to let your friends know about #77.