I hate second guessing the dealer, but there are a few things that I've seen.

First, possibly the internal configuration of the radio may be set up wrong. Whenever there is an addition of components, there is a programming procedure that must be done to make the system work correctly. Possibly this was done incorrectly or not at all. I've seen this botched even by Mercedes. Was the additional components (CD and Phone) installed prior to being delivered to the dealer, or dealer installed? In either case what needs to be rechecked is called the fiber optic configuration.

OK, another thing it could be is this, and I know it sounds strange:
On the sides of the CD Changer are adjustments to set the CD changer suspension to either horizontal or vertical. If this is set incorrectly (incorrectly on your SL would be horizontal, it should be vertical) it can, believe it or not, cause this exact concern (so can the previously mentioned possibility though). Doesn't make sense, I know, but it can cause this message, especially on the latest style cd changer.

One final thing is the routing of the fiber optics cable. If the cable is bent at too much of an angle it can cause all sorts of problems. On the SL (129) installation what is supposed to be done is a small "door" or "flap" is cut into the left side trunk trim to allow the fiber optic cable to go from the inner fender area, though the trunk trim and to the changer (and in your case the phone components). If this isn't done properly, like they try getting the fiber optics cable to these components some other way it can damage a fiber optics line. With all that phone stuff back in there, it's easy to get a kink or pinch in a fiber optics cable.

I would suggest they do the simple thing first, which is to double check the configuration in the radio (D2B component configuration), then if that's OK maybe check the cd changer suspension setting, and when they have the changer out to check that, look and see if any of the D2B cables are too tighly routed, and see if the cable was properly routed through a flap that should have been made in the trunk trim.

Gilly