I have the same Siemens BIT II phone in my '02' 745i.

In IDrive :

Goto Settings (diagonal bottom left)
Select 'More vehicle options' (ICON looks like side of car)

Then make sure the 'Tel.Base' option is selected (should be highlighted in orange which is ON , white is OFF)

This then matches the phone with the car. Make sure you have a working SIM card too.