I recently e-mailed the Royal Parks to complain about the use of an 0843 number displayed online for anyone to call about any Parking issues. I pointed out the costs to them, and was e-mailed back to advised it had been passed to NSL Services (who provide the parking).
I have been busy and hadn't followed it up, but I just check and to my amazement, the number is now an
0343 number! Next time I am at Greenwich Park (my nearest) I will check the number they have displayed for help.