Hi there, the only 2 phone options for your MB are of the TDMA, and CDMA versions from Motorolla. They are the only 2 phone options that can be installed in your C-class that will allow you to utilize the full feature of the hands free option and use of the mutlifunction steering wheel for making calls,ending calls and voice control. Mercedes Benz uses fiber optics for their radio,cd player and telephone support electronics, unfortunately not every manufacture out there has products compatable or able to merry up with the fiber optic loop.
As far as for why there is only 2 options for phones it may be due to price, infrastructure to support gsm technology in the conus, or possibly Motorolla is unable to put toghter a package that is cost effective.
This does not mean you cannot put a gsm phone in your car,you can buy an adapter where you can proabally plug it in your cigerette lighter and the other end into the phone where you have a speaker phone,hands free option;however,you will not have the full functationality of a timeport phone with the electronic support for your specific vehicle.I have seen all different types of phones installed in cars from different companies,you will not be able to utilize the features that consumers that purchase a MB phone for their cars are able to retrive.Hope this helps.