The phone in the picture is this
http://bmwfans.info/original/E53/SAV...4/ill-84_0104/
SIEMENS BIT II
As I told you, I think it's a bluetooth
but only between car and cordless handset.
You can remove it and proceed with a ULF Bluetooth RETROFIT that will "PAIR" with every bluetooth compatible that you are using.
Check for BLUETOOTH RETROFIT in the FORUMS if you are not satisfied with your current status and want to upgrade...