Palm has released an official patch for this Bluetooth problem: when using BT car kit or headset, if a second incoming call is received and ignored, the audio of the first call is lost.
Your Treo Pro needs to be version T850UEU-1.01-XXX or lower, newer versions have the fix already.
More information & download: Palm Support Case 2457
Palm Support Case 15525
Update: here's a direct link to the CAB file: TreoProBTPatch.cab