OK, I did some research, it appears a GSM phone is a type that isn't used here is the US yet, but is not a satellite phone. It looks like it is a different antenna though, check out this link, look at option code #270......
http://www.imtcexport.com/Resources/AllMBCodesShort_TITLES.pdf
A visit to this site shows some antennas being offered that seem to be just for GSM:
http://www.aerpro.com/download/Antenna_May2000.pdf
Here is a really useful link to a guy who it sounds like is going through what you are, except possibly not in Hong Kong:
http://srd.yahoo.com/S=2766679:WS1/R=7/K=mercedes+ml+gsm+phone+antenna/H=0/T=1041773170/F=1b90a06ccda6c283b959fee8650a8dd1/*http://www.keysolutions.com/M-Class.nsf/aac7d56ca8fd884b852563be00610639/AD96201628AB6EA685256921000856F8?OpenDocument
Well, that's about all the surfing I feel like doing on this for now, i hope there is a "silver bullet" in all of it for you, especially that last one, go down about half of that page and you'll find where he installed his phone, lots of good info there.
Gilly
ps: decided to add this paragraph from that last link I posted. This seems like the most important part to read, but it is an interesting link in it's entirety:
Since I was also installing a GPS system, I opted to get MB part # 163 820 06 75, which is the combo GPS/cell antenna that made its debut with the 00MLs. The cabling in the 99s (and the 98s, I suspect) is RG-58U coaxial cable, which support analog, and the various flavors of digital (i.e. CDMA, TDMA, and GSM). Since I have a 99, I also had to get MB part # 163 820 12 89, which is the complement to the combo antenna. The antenna muxes the GPS and cell signal together, while this part splits them back out again. This splitter took forever to hunt down with zero help from MB--the only reason this project was completed was because of great help from my dealership's parts guy and service tech. For model year 2000 and beyond, you already have the splitter and it is located behind the radio--astonishingly, it is clearly labeled "splitter".