Can a Mac be set up to dial arbitrary numbers by connecting to an iPhone?
I use Google Voice's service to have my computer ring a third number and call my iPhone. Since I have most contacts in my Address Book and don't mind adding new ones - iCloud will get most contacts to the phone and Siri does a nice job dialing things, but there are also software packages that will literally dial your iPhone over a bluetooth connection.

Check out Dialectic for starters if you want a tool cut exactly for the job of dialing from OS X to iPhone remotely.