Voice cuts out in car on bluetooth iPhone

Forgot to add..this only happened since 4.2.1 update, before it worked fine with the car.
Mac Pro Mac OS X (10.5.4) MacBook Pro, Macbook, PowerBook G4

A simialr problem for me with the Bluetooth in my SEAT Leon. I can make and receive regular calls but if I try to dial voicemail (regular, not visual) I hear it connect to the voicemail service through the car but then it drops the audio from the car and continues the call on the phone (without any keys being pressed on the phone or on the car). The iPhone (in this case, a 3G running 4.2.1) still believes that the audio is being routed to the the bluetooth device though.
iOS 4

Similar things are happening in the builtin Bluetooth Handsfree on a new Audi A4.
When the iPhone calls voicemail, even if the phone indicates that its sending to bluetooth, nothing is heard in the car.
Any other number works. This is regardless if the voicemail button is pressed, or if the number for the VM-services is entered (as iPhone recognizes it as VM).
This was not a problem in 4.1 or any previous sw, so it seemed to appear on 4.2.1. Have not verified its the same with my iPhone 4, but at least 3GS has this problem.
iP(hone 4ad) iOS 4

Further testing has found that when dialing vodafone voicemail (121) the phone cuts out. If you dial the long number for vodafone voicemail (07836 121 121) then it works OK, albeit that you have to key in your mobile number and pin. Seems to be an issue with iOS 4.2.1 and dialing short (i.e. 3 digit) numbers?
Mac Pro Mac OS X (10.5.4) MacBook Pro, Macbook, PowerBook G4

Same problem. I have iphone4 with Audi MMI system. Was all fine up until recent. Though it was working at first with 4.2.1 update but not sure.
Can call 121 using the MMI system, it phones 121 then immediately disconnects from the car but stays connected to the phone.
Anyone got a solution? is it possible to go back to previous phone software version?
Phil.