To change your dial-up number in Windows XP follow the steps below
1. Click on the 'Windows' button.
2. Click on 'Control Panel' Change your view from category to either small or large icons.
3. Click on the 'Network and Sharing Center' icon Click "Change Adapter Settings" found at the top of the left column.
4. Find your current dial-up connection in this list and double click it. This icon may have the same name as your current internet service provider, or it may just be called "Dial-up Connection"
5. On the screen that opens, you will see the dial-up number in the middle. Fill in the area code, the phone number and then click "OK".