Although don't try a similar trick with the 'How's My Driving?' stickers on the back of lorries... the number you dial is probably that of the driver.
Not personal experience honestly!
My favourite occupation when dragged out kicking and screaming to go shopping, is to go into a high street chain store, loiter without intent, and when the sales assistant approaches you with "can I help you", I answer "with what". This is a sure way to find out who sticks to their script.
As regards lorries, yes, calling the number up does work. I reported a driver from Somerfield who drove through a red light in a traffic lane to go straight on and turned left.
He was susequently fined by his employers and suspended without pay for 2 weeks. Bearing in mind the carnage he could have caused, he got off lightly in my opinion.