To change your dial-up number in Windows XP 7 follow the steps below
1. Click on the 'Start' button.
2. Click on 'Control Panel' In the left column, select "Switch to Classic View".
3. Click on the 'Network Connections' icon
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.
4. On the screen that opens, your current dial-up number is seen at the bottom. Change this to your new number (be sure to include the area code at the beginning).
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".