We also give third parties the content of our calls as well. Under the third party doctrine, it's then legal for the government to listen to the content of our calls. The reason they claim they don't collect all the content of our calls is that they know people would be up in arms. But it's perfectly legal for them to do so according to the third party doctrine.
So I actually think Katz, which held that wiretaps were a 4th amendment violation, was wrongly decided, or at least makes no sense in the context of packet-switched VOIP. And I think that for the reason you point out: it's hard to reconcile Katz with third party doctrine, at least when AT&T has digital copies of your calls.
But ultimately this is the problem with inventing "privacy rights" out of thin air and duct-taping them to the 4th amendment, which has no such broad concept of "privacy." There's no broad principles to rely on, just case-specific hacks.
