Phone Still Rings After Answering

[C3-PO]

I have no idea what is causing this, but when I have an incoming call, sometimes it keeps vibrating after I've answered the call. So while I'm trying to talk to the person, the phone keeps vibrating in my ear. Very annoying.

At first I though, since I have Hangouts Dialer installed, that my regular phone app and Hangouts Dialer were both trying to be answered. So I uninstalled Hangouts Dialer. But it still happens. Does anyone know what this could be?