Disable call waiting

Why would I want to turn this off?

1  In the taskbar, click the Windows Start button, point to Settings, and click Control Panel.
2  In the Control Panel, double-click Modems.
3  Select your modem from the list and then click Dialing Properties.
4  In the How I dial from this location area, check This location has call waiting, and then type the number that disables call waiting.

How can I find the number to disable call waiting?