Support - Dialup - Change Access Numbers In XP

How to Change Your Local Access Number in Windows XP
  1. From the Start menu, click Connect To, and choose your SAMnet connection.

  2. Click the Properties button.

  3. Delete the contents of the Phone number field, and type your new access number.

    Note: Remember to check with your local telephone company to make sure that the number you choose is a local toll-free call.

  4. Click the OK button.

  5. Click the Dial button to connect using your new access number.