I was employed for over thirty years in the road transport industry ( not driving) until my retirement in 2008, I used to see on the back of vehicles notices to comment on the driving ability of the operator and a freephone number to comment,why did they disappear .
We still have them, still see them about.
A bloke I know once got pulled into his office because he let a woman out from a junction, she phoned the number to say he’d let her out, thank you, blah blah blah.
He got a bollocking because it cost the company £40 for every phonecall and she’d called twice, ‘to make sure’. 
The stickers were removed shortly after that.
A.
I’m guessing they would be to expensive to keep up and running, I bet people we just phoning them looking for a freeby or some vouchers 
*** Removed because I didn’t read op properly, sorry ***
A mate of mine had one on the back of his ( own) Sprinter which looked all official but said “How’s my driving? Dial 0800 EAT ■■■■ AND DIE.” Crude but effective.
I rang a driving instructor up I was following who was using his phone without handsfree (he was driving). I told him of the law. He hung up on me and then gave me the 1% rise in his interior mirror.
Still a few around but years ago a few like minded souls changed the numbers subtly or went the whole hog and stuck a new number on for a premium rate ■■■ line. Trouble was I had to keep looking at the back of me truck to call it and its just not the same in a phone box.