So far I've dialled 999 (or UK varient of 911) 3 times in my life.
1st
In rented accomodation about 8 years ago my front door was "flat packed", essentially completely taken out, frame and all by a gang who hit the door that bloody hard it all caved in. This was about 3am or something similar. They rushed through the house looking for my car keys to my Subaru Impreza I had parked outside. It was the early beginnings of not stealing cars as such, but stealing car keys from inside houses. The thugs (6 I think) ran into my room and held me at knife point demanding the keys to my car.
Obviously I gave them ! Immediately they were gone, dialled 999 and they came back, ripped the phone out and took my Denon Stereo stuff.
Called 999 from my cell and in no more than a couple of mins there were police EVERYWHERE. Chopper searching and everything. I was VERY impressed.
The car was recovered some 20 miles away totally undamaged and containing a LOT of very expensive clothing. Police raided their house and recovered my stereo equipment etc... also recovered a BMW M5 and a badly damaged Range Rover. Ran the car for a couple of more years in competition before selling it.... moved house and bought somewhere with a garage !
2nd time
Following a car home from my parents it was clear that the driver was DUI. Called 999 and explained the situation. Gave a commentery of the driver's antics. I eventually passed the DUI car as advised by the Traffic unit heading towards me. Advised to pass and flash at the traffic car to let them know they were approaching car... Very strange flashing a police car going the other way with it's blues on
Got a call back from the Traffic unit to let me know that the person had been arrested for being DUI approx 15 mins later
3rd time
Driving up the motorway (UK M6) came across a person walking between the central armco between the 2 fast lanes
Immediately dialled 999 and the Police dispatcher said they'd had lots of calls about this person but no-one could give an accurate location. Gave a location (to within a few hundred yds) and a short while later met a number of traffic units heading the opposite way/