That's identical to the situation in Leicester! (Even the number is the same: 005!)
I can dial numbers in the Leicester halls from my office, but not from home. It too is a switchboard system, with '9' for an outside line.
Some people reported that they can call these numbers from NTL phones, and from Orange mobiles (which use the NTL network). They suggested my office (which is in the university) has an NTL phone, but I don't believe it is. It predates NTL by a long time. They then suggested that the university might have some sort of link to the halls, but I don't believe that either. As far as I know, the systems are entirely separate (there's no way for students to dial university numbers for free either).
Presumably you don't work at the University of Nottingham. In that case, who provides your work phone network? If we can figure out why these exchange system allow us to dial these 005 numbers, we could be on to something!
Update: I've just tried a Nottingham number on 0115 005 (chosen at random
) and got through to some poor chap who sounded like I'd got him out of bed! So it's definitely
not a link between Leicester University and Leicester halls. I'll see if I can find out who provides the campus phone network.