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.