Hi,

As far as I know, changing the call UI is not possible as this is a closed source component.

However, the closest thing I could think of is create a script that when enabled would listen through Dbus to incoming calls, and once accepted would turn on FMTX. I know this doesn't solve the problem with forgetting to turn it on once in the car, perhaps a http://wiki.maemo.org/Queen_BeeCon_Widget widget would help.

Cheers