OK so I just recently switched over to a 8900 curve from the bold a couple days ago.
Everything's great except I noticed one thing that really bothers me, BB messenger doesn't work during a call is in place. The Bold had no problems with this.
Is there something I can enable or do I need to update the firmware. I'm using the stock default firmware right now which is .94.
I would definitely upgrade your OS to one of the more recent ones, i didn't even know there were still Javelins out there on .94. Try that and see if that helps, I'd suggest .168 or higher.
The way the network functions is the culprit. You will just have to wait until your are off the phone or, as Warwick says, use WiFi to receive BBM messages whilst yakking.