I've tried searching for the similar topic, but didn't find one.
There is this annoying problem with Bluetooth. While connected to bluetooth car kit and making a call everything works fine until you want to go private and switch to the handset speaker. You may here some sound for 5-10 seconds and then you can hear nothing while the other party hears you. Switching back to bluetooth resolves the issue.
I could post a bug report, but have no idea how to do this