Hi.
I suppose you are reffering to the 'full duplex' property of MMAPI.
This is not possible (AFAIK and I have researched the subject). The J2ME does not support full duplex audio recording/playing (although you can do that in the BB's simulator it does not work on the actual device). So unless you have a support from the manufacturer via API (or any other way) you cannot do it.
And no one has done it so far.
One company - agito: BB VoIP - claims they did it, but offer no proof of it - so we can assume the same marketing trick (as will be explained next) is in play)
The VoIP solutions aren't real VoIP. No audio is actually carried over data channels.
What they do (if you read carefully through the datasheets and official forums) is they use a callback.
How this works?
1) You initiate a 'VoIP' call.
2) The app informs the server about your desire to make an outgoing VoIP call (it uses data channel for that - whichever you have available)
3) The server calls you back and the app (or you) answers the phone
4) The audio goes over normal GSM (or whatever) channel and is spendingmoney 
So, as you can see, the advertised 'VoIP feature' is actually a scam (at least as far as BB is concerned).
VoIP is what it is: a Voice over IP. Since they don't send audio over IP, they should not market it as such.
Hope I helped,
Myraddin
