Equipment:
Mercedes Benz MB SD Connect C4 Star diagnostic system (SDS)
Dell D630 laptop

Who need to enable Mercedes Benz Telephone buttons on the steering wheel?

If you have changed out form an Audio 20 to an aftermarket head unit with a steering wheel control module for a phone, a keypad on the centre stack, and a telephone function button.

Instructions in DAS Developer Mode:

Install and activate the software. Software is pre-installed and activated if you get the MB SD C4 multiplexer and Dell D630 laptop as a bundle package
Connect MB SD Connect C4 diagnostic system with car via OBD port
Run MB SD C4 DAS software
Select vehicle model
Select Control units groups
Select Information and communication
Select ICM Instrument Cluster
Select Control unit adaptations
Select Customer specific settings
Press F2 to accept the SCN prompt
Change IPOD: Operation via multifunction steering wheel to POSSIBLE
Press F5 to save the changes to the car
Press F3 to start the transfer
Press F2 when precious procedures are complete
Click OK to reset message
Click F2 back to Control unit adaptations menu

http://blog.mbstarshop.com/2015/02/10/activate-benz-telephone-buttons-on-the-steering-wheel-with-mb-star-diagnostic-tool/