Telematics Control Unit. If you intend on keeping BMW Assist then you'll need the TCU with bluetooth. They are not cheap and will need to be coded by the dealer to work properly. What a lot of people seem to be doing is that once their BMW Assist subscription expires they choose not to renew it (since 99.99% of people have cell phones with them at all times) and by the BMW bluetooth module called the ULF which gives you everything you need except for the BMW Assist functionality. It also does not require coding by BMW so it's more of a plug and play option. See this site for all the info you could possibly want on it. Bimmernav is top shelf and a sponsor of this site I believe.

BMW Bluetooth X5 retrofit - Bimmernav BMW Upgrades