I thought I'd come shortly back to this topic. It took me some time to purchase the wiring harness and the MOST fibre parts. I ordered these parts from MBenzNL mainly because I didn't want to pay ridiculous price our local dealers ask for spare parts and also because I was not sure about the best MOST fibre parts. The 221 without VCM has the fibre without connectors down to the amplifier at the trunk while a VCM car would have an extension connector at the rear fender area.

I bought the W203 wiring harness, pulled microphone connectors off, equally from the original telephone control unit connector and moved microphone wires to the new connector.

I would have had enough fibre to install the VCM as from the factory to the bracket at the trunk. That would have been the VCM connector facing backwards in the car. Instead I turned the VCM 180 degrees to make the connector face forward. This way I was able to plug the fibre that used to go to the phone connector module directly to the VCM without an extension cable (one fibre connector less in the loop).

I was able to code the phone control unit for "microphones not present" and the MOST ring configuration as present after the VCM installation and everything started working properly. One more task remaining though, I have to have the VCM language changed, my German skills are sufficient to test the different functions, navigation, phone, radio etc. but I really have to have the language changed to English for (more) fluent use. This can only be done with SDS on-line.

While swapping microphone wires from one connector to the other I observed a deviation from the WIS wiring diagram. I wonder if anyone has any experience if it makes any real difference but for some reason microphones 2 and 3 were swapped at the phone connector. My parts supplier told me that they have seen other cars with similar mismatch between WIS diagrams and the actual wiring at the car.

I had done quite a lot of homework and would probably have finished the task on my own but big thanks still go for this forum and MBenzNL, it is a lot easier to work at the car when experts confirm your assumptions and give further guidance.