3g unable to make calls . period iPhone

Ok. heres the situation, and before i start, no. im not whining about 3g reception or any of that usual stuff.
I have an iphone 3g i got launch weekend, and i got my girlfriend one a few weeks after launch, the exact same model as me. (The 8G black).
I have had no real issues so to speak with my iphone at all. Despite some minor annoyances with itunes and it suddenly not recognizing the stuff on it and requiring me to wipe and re-sync it from scratch.
But for some reason my gfs iphone can not make phone calls. Like at all. She dials. and it rings. then cuts off before anybody can pick up. Tries multiple times in a row. Full reception bars. So she turns to me. and makes the same call on my phone. we are sitting in the same room mind you literally feet away from each other, and the calls go through and she talks for an hour no issues.
The same issue is present for calls she receives. It rings, she picks up and then click. gone... No talking at all!
This has happened literally every time she has tried to make a call. Tonight for kicks i had her switch 3g off and use edge. which does seem to work for talking.
If this were something thats going to be patched and fixed up, i could see saying sure just wait for them to fix it. But this is something that has been said would be fixed for like 3 patches now. (We are both using the latest 2.1 patch)
I am starting to think that the phones themselves are defective. How can my phone which is the same model, same everything, same service, standing in the same room. Behave so differently. One works, the other doesnt. We are at the point now where she wants to take the thing back.
Has apple been replacing phones with issues as severe as this? How would we go about doing this?
Thanks!
I love my phone and I guess i should count myself lucky to have one that works!
....pc Windows XP Pro