NTL grew rapidly by buying up lots of smaller providers. With those
purchases, it inherited multiple billing systems - all incompatible. There
was a project ("project Harmony") to provide an integrated billing system
on top of the various stacks they had, but that was so bedevilled with
politics, I've no idea if it ever got off the ground.
I don't know if things have changed much in the last few years, but it
always used to be the case that the development team were in continual
fire-fighting mode; what got implemented was always put together within a
few days of it being needed...
