Does anyone know who does such a modification?

There are a few examples displayed in the Member MINIs section...

 

My mini has a little microphone opposite the sunroof control. I want to use this... where do I inset my SIMM? And will T-mobile give me another one.

 

any installation requres addtional equipment. The built-in hands-free (roumored to use the center left button on the multi-function steering wheel) is not availalbe yet.

There are plenty of 3rd party installtion jobs you can get. My favorite that I have eyed is the Bluetooth Kit from Sony-Ericsson, so I can use my phone even when it's in my gym bag in the boot!