If you've been renting a modem from InsightBB, and you replace it with a different (non-rented) modem, InsightBB will bill you for the rented modem until you return it to them. They will not* prorate the rental fee as of the day you switched the modems. So, it is in your best interest to return the modem to them as soon as possible.

* Well, the author was able to get a prorate without asking, but this was after a billing snafu a few weeks later. More details in his connection diary - /dsl_diary/other/408497 and look at the entry dated 2003-04-07.