See also this.

The point is, what part of Please do not Climb the Monument signage are those who still climb Silbury not understanding? It shows nothing other than a selfish disregard for Silbury’s long-term conservation. As for climbing Silbury and then phoning people from the top... hardly something to brag about on a board where most of us respect such places and have their conservation first and foremost in our minds. More puzzling is actually wanting to call people from the top of Silbury! Would it be the usual banal, “I’m ten minutes away, where are you?” Or perhaps, “I’ve forgotten the drink, can you bring it up with you when you come?”

Let’s hope English Heritage capped the hole properly; we wouldn’t want people falling down it would we... can’t be much of a signal down there ;-)