Follow these steps to change your dial-up number in Windows Vista.
1. Left-Click the Windows Logo at the bottom left of your desktop.
2. Left-Click ‘Connect To’ option.
3. Right-Click on your Dial-up Connection and Left-Click on Properties.
4. Left-Click the ‘Phone Number:’ box and delete the number and type in the new number corresponding with the number for your area.
5. Left-Click on OK
6. Close your ‘Connect to a network’ dialog.