According to Palm: "If you are in an active call using bluetooth car kit or bluetooth headset and you receive a second incoming call and you let it ring or you ignore the incoming call, the call will be diverted to Voicemail. The first call remains active but you can no longer hear the caller (the caller can still hear you). The audio has been lost. If you answer the call and swap between calls or reject the call, the issue is not seen."
Palm said that the patch is only for Treo Pro's with software version T850UNA-1.01-XXX or lower.
Download the patch at the Palm website.