Originally Posted by KiwiJochen

Time to compare TCU wiring:

USA TCU Everest
54p black connector.
Key pins:
15=ibus
17=Term30 (permanent power)
18=Term30 (permanent power)
35=TermR (ignition switch power)
36=Ground
52=Illumination
54=Ground

Euro ULF
54p black connector.
Key pins:
15=ibus
17=Term30 (permanent power)
18=not used
35=TermR (ignition switch power)
36=Ground
52=Illumination
54=not used
== same as US TCU

BIT interface with Siemens phone
54p black connector.
Key pins:
13=ibus !!!!!
17=Loudspeaker !!!
24=Ground
25=Term30 (permanent power)
31, 32=Term 30
33,34 = Ground
42=Ground
43=Term30
== MAJOR wiring differences between BIT wiring and ULF/TCU

CONCLUSION: Your loom is wired for a different Telephone module.
Because you don't know what your loom was made for the ONLY solution to your problem is to pull the connector housing off the loom's telephone connector and document all fitted pins and wire colours.

THEN re-pin the connector to match your telephone unit that you are fitting
(Re-pin = remove wire from existing connector pin number, and insert to new pin number. Use small jewellers scredrivew to push down the locking tab to remove the wire.)

Refer to the WDS for your correct pin numbers and wire colours.

After an hours or so of repinning you'll have the problem fixed.