I am a current Nextel user and will be receiving my A5 this upcoming September. Don´t have any experience in this matter, but I can tell you that a friend of mine has the same problem as you.
This friend of mine who has a i880 (Nextel) told me that he can answer and hang up calls by using his steering wheel, but can´t manage to synchronize his contact list with his screen. He also told me that he can´t figure out a way to use the two way in his car.
You are not alone.
Truth of the matter is that I don´t believe the 2 way will work. You have to take into consideration that the people who developed the car don´t share the IDEN technology that we have here in the americas.
Nonetheless, the dealer is getting me a demo A5 to try out next week. I will give it a shot at connecting my phone and seeing whether it actually works.