Does the telephone bill vary inversely, directly or neither as the number of telephone calls made?
Here in Canada, the answer is neither; the telephone bill is normally based on a flat rate plus long distance charges which, in turn, are dependent on duration.
If in New Zealand you are charged a flat amount for each phone call (regardless of distance and duration) and there is no base cost, then the bill would vary directly with the number of calls. Otherwise, the answer would be neither. (I can't imagine that it would vary inversely, since this would imply that you could reduce your telephone bill by making more calls).