Many, many times.
First two occassions to report drink drivers I was behind. Next was a ****** boy racer who reverse-handbraked his Nissan Micra onto it's roof. 5 occupants smoking, petrol everywhere etc.
Then I joined the military. In the mil, 999 is 222. The mil equivalent. Bit if you phone 222, you generally get a retard answer the phone, so it was quicker and easier to dial 999.