Microphone "speaker mode" sensitivity with headphones plugged in?

In my car I have the headphone socket of my Galaxy S2 plugged into my car’s auxiliary socket so that I can listen to music/podcasts whilst driving.

This would also be useful for handsfree calls – except that when someone rings, I can hear their voice through the car speakers, but they can barely hear me. If I press the “speaker” button, then they can now hear me perfectly, but I can’t hear them.

Is there any way to have the microphone sensitivity of “speaker mode”, but with incoming audio still going through the headphone socket?

If not, does anyone know of any hands free alternatives without spending a fortune on a bluetooth hands free kit?