I am using N900 since 2 years and have no issues so far. I have 2011 Jetta and using the Bluetooth with no phone book and no issues.

From the link
http://wiki.maemo.org/Bluetooth_PBAP, I have updated my phone for the PBAP as exactly mentioned. I do see the Phone book in my carkit and I can make the calls but the biggest issue is

"The other side can hear me, but I can't hear them and also I can't hear the phone ringing sound too. But when I receive a call, I can hear the ring, but I can't hear from them."

Can you please some one help me if I need to updated anything to my N900 or at least let me know, how can revert back my N900 before this update. I am fine living with no phonebook in the carkit as long as I can get the audio in bothways.

Your help is always appreciated.