The Google Voice iPhone app found a very clever way to make calls.
When calling someone, it used to first call you, you'd answer and then it would ring the other person.
Now it seems to make a request to the GV servers and dynamically allocate a phone number to your target, then it tells your iPhone to dial the temporary number which they forward on to your target's phone number.
The end result is seamless! Go to the GV app and dial a number, and it Just Works™