To change your dialup access number open your windows dialer.  It looks like this:

If your unsure of how to open the dialer, follow these instructions:
Click Start
Click Control panel.  It may be listed under Settings if you have not updated to Service Pack 2.
Make sure you are in "Classic View"  This is the one that does not say "Pick a Category" at the top.
Double click on Network Connections.
You will see an icon for your dialup connection.  Double Click on it.

Now that you have your Dialer open, all that you need to do is to put a new phone number in the same place as the old number is now.  Just click in the box delete what is in there currently and add the new number.  A list of our current access numbers can be found at http://support.logical.net/lib/signup/dialup_number_lookup.asp