iPhone suddenly asks me to press 1 to accept calls. how to stop this?

In the past few days, iPhone requires me to press 1 to accept calls. I have made no setting changes. This behavior is totally unacceptable, especially in the car, where I don't have a button to press on my navi screen—I was always able to just press "accept" to talk hands-free. In searching around, the only comment I see refer to Google Voice. I do not have Google Voice, and don't even have a Google account! Why is this happening, and how can I make it stop?