You should be able to use Comand to find your contacts once transferred, the steering wheel buttons to receive and make calls if your 204 is anything like my W211.
Yes, seems to be a similar set up to my E55 but I needed the Viseeo MB4 unit to link to my phonebook/contacts, well that's what I thought but from what you're saying I may have wasted my money!!
On my W211 (E320) i have the mobile phone prep and the connector in the arm rest. due to cost I found a cradle/adapter for a Nokia 6020 on ebay, a friend at work gave me an old and tatty but perfectly functional Nokia 6020 I added a GiffGaff PAYG SIM and it works great. I don't mind that it's not my regular phone I only use the car phone on vary rare occasions and the only my family have the number.