(I'm catching up on old mail, after being on vacation)
I worked at an ISP in the Detroit area for a few years, and the seasonal
dial-up usage variations were pretty dramatic. Through the spring and
summer, as the weather got nicer, we could add large numbers of users and
still see the modem utilization stay pretty steady or decline. On the
other hand, in the fall and winter, when it was getting rather unpleasant
to be outside, we were having to add capacity pretty constantly, even if
we weren't adding many users, in order to avoid busy signals. In addition
to the capacity that ISPs have to add at that time of year, I assume the
CLECs catering primarily to ISPs probably have to make similar adjustments
to their trunking capacity at that point, although they probably see that
far more closely tied to the number of lines the ISPs are ordering from
them.
It wouldn't surprise me if non-modem and non-internet phone line use goes
up dramatically in that part of the country during the winter as well, but
I don't have any actual data to support that.
-Steve