I have been through this regarding my '99 SL600 phone as well.
You can get a new Motorola digital to work with the hands free system from MB, HOWEVER it costs $1,600.

I have found a place in the UK that sells a harness adaptor which attaches to the cradle where the original phone plugs in. I'm now looking for someone with enough knowledge to build the necessary interface to a current digital phone.

Anyone interested in working with me on this please contact me. There should be a lot of owneres interested in solving this problem to make the customization work more affordable.

Ron
NYC