Follow these steps to change your dial-up number in Mac OSX.
1. Click on the Apple menu then System Preferences.
2. Click on the icon labeled Network.
3. Select the modem port that you are using. Most computers use an Internal Modem.
4. Click on the PPP tab.
5. For number, type in the dial-up number that is a local call from your location.
6. Close the network configuration window.