I looked into this when I got my P38 and I think the consensus is that the Harmon Kardon set up and general P38 complexity makes this very challenging (read expensive).
As the previous poster says you can do it if you have a separate speaker for phone calls but I ended up going down the route of a device that clips on the sun visor. The one I bought is very discreet, can pair with two phones at the same time and reconnects to the phone when you get in the car. Mine's a Supertooth Crystal I think. Battery lasts for a month, and if you have an iPhone it shows the battery status when connected. Good device and was about £30.