I just installed a CK20 car kit and found that my Navi wheel call and hang-up buttons didn't work, and the Voice Dial button makes my phone re-start. Then I remembered that I installed Conversations which changed a lot of the contacts software on the phone so I swapped phones since I had a spare new one at work and now the Navi wheel buttons work.
Just a warning, installing conversations may make your car kit Navi wheel buttons useless.
It is possible that another one of the Beta's broke this connection but I figured that Conversations was the one that did it.
Nokia 6120 Classic FW 5.11