I have same problem after updating from original firmware (11) to version 21 - My headset also worked fine just before upgrade.
If anyone comes across a fix for this please post it as it's a real pain to stop my bike to answer the phone even when I am wearing the headset.
Everything else much improved, gallery loads quicker, much more RAM and GPS locks on fast enough for me to actually use it (shame about loosing the track position on route though).
Peter
N95,K800i,Apple Powerbook.