Use speakerphone mic (i.e., high gain) while headphones are connected

With a Galaxy Note 2, when answering a call with headphones (but not a headset) connected, the audio output goes through the headphones and the inbuilt microphone works as though it were a normal call (so it's too quiet when it's a few inches away). When I switch to speakerphone, it ignores the headphones entirely and audio output switches to inbuilt speaker. Basically, I'd like to talk to people with my phone on my desk and a pair of conventional headphones.