All MMI based Audis (well, considering pre-3rd gen anyway) that come with Bluetooth phone-prep include two modes of voice control (either will be enabled depending on market and car settings a maintenance shop or VAG COM user can adjust):
1) Phone voice control with dialogue. Language can be selected by maintenance shop or with VAG COM. Limited speech dialogue to control phone functions.
2) "Speaker dependent" voice control without dialogue. Very limited mode that is language independent in that it only recognizes user-programmed voice tags and answers with beeps.
Only exception is some U.S. Audis that have the phone speech control disabled, but it too can be enabled with VAG COM (it is just flipping a bit on). All (current anyways) MMI phone-prep cars come with both of the above in hardware, either (or neither in some cases, e.g. if the third option below is installed) is enabled via software configuration.
Third kind is available when the car is fitted with speech dialogue system:
3) Speech dialogue system with controls for phone, radio, CD, navigation, directory. Correct language module must be installed in car, only one language fits inside the module software at a time (upgrade CD can be used to install new language). This is a module inside the radio module in trunk, very easy to retrofit too.
For the VAG COM options to configure either of the two first options here:
Interesting VAG-COM codes for Audi A5 - probably A4 B8 too, maybe A6 etc. as well
http://www.geocities.com/a6retrofit/articles/a5vagcodes.html
For retrofit instructions (A5 with navi is same as A6) for full speech dialogue system see end of this article:
Bluetooth phone prep and speech dialogue system retrofit Audi A6 (4F/C6) Avant MMI High (possibly relevant also for Q7/A8)
http://www.geocities.com/a6retrofit/articles/a6phoneprep.html