The initial scaffolding to test the "on call" part of the dialer is getting in place (see bug 902983). One area of the code not tested yet is the bluetooth support. Since we rarely test this during our manual tests it's a great area to unit-test.
I have the feeling your hard work reviewing bug 902983 will put you in a great place to tackle this one Rik :)
This was done as part of the callscreen split :)