I'm surprised that ZXR states that the phones go through to the Highways Agency because, whenever I've used them, the operator on the other end has always said something like 'police emergency' or 'police control room'.
There must be a lot of ladders about on our roads because, like ZXR, I've seen one. It was about 15 foot long, aluminium, and lying precisely positioned along the white line between lanes 2 and 3 of the M25, in the rush hour during torrential rain! Hundreds of car drivers must have swerved to avoid it but I was the only one to stop at the next roadside emergency phone to report it!
Using the roadside phone has a slight advantage over dialling 999 (or 112) from your mobile. You can simply report the number on the phone (and, for example, state that the ladder was about half a mile back) and the police (and Highways Agency) will then know exactly where the problem is.
Chris