This sounds more like a software issue in the Bluetooth carkit, are you having the carkit speak incoming caller ID's? how are they connected, is the bluetooth connection between them "always on" or only initiated on incoming calls?

Sometimes the phone might want to transfer the call and the carkit reacts too slowly, sadly mixing brands like this increases the chance of problems, especially with the pretty weak Bluetooth implementation Siemens has.

How are they bonded, as headset or as handsfree device?