I mixed it up and GP is right. But you are also right (which illustrates the point GGP was making)

If you and I are in a phone call (the target of the phone call located in a one-party consent jurisdiction), I can record it or you can record it without telling the other: that's one-party consent, because one of the parties of the call consents to it

At first glance that may sound pointless, but it means a third party outside the call (e.g. phone company) could not record it without one of us consenting

By notifying that the call is being recorded, that now can create two-party consent because you are still on the line (and thus implicitly agree to the recording, but as you and GGP point out that's not always considered consent enough) -- one-party consent would have been if the company on the other side of the line simply recorded the call without telling you