Hello and welcome.
First of all can we establish that you definitely have the necessary phone pre-wiring.
Obviously don't know how good your knowledge of Mercs is, but nearly all have the phone buttons on the audio and steering wheel, but that doesn't guarantee that they have the pre-wiring as it was a cost cutting exercise to install the buttons in all cars, even without the rest of the equipment.
Do you have COMAND or standard Audio?