UllaBella

UllaBella is a plugin for the Address Book in Mac OS X that makes it possible to dial any number with the internal modem found in most powermacintosh computers today.

When the installation is done, start the Address Book. You should now find a new item in the context menu when you click a phone number, saying "Dial with internal modem". Pick up your phone and click that menu item. After a second or so, you should hear the modem dialing the number for you. Now have a nice chat with your friend.