I didn't do any extra wiring at all, just used the incartec harness for steering controls for the 350Z for use with Pioneer HU's.
To be honest, I don't know if the call answer button works as a call answer when there is an incoming call, usually just let it auto answer.
I just know that the call answer button acts as a source switch for the HU when not in a call.
Also, I found that Sextons were selling an outdated version of the incartec harness, so I ordered it directly from incartec and they sent me the correct one.
Many years ago now, but still working like a dream