If you have a Treo Pro you may be interested in this quicky patch from 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.
We have seen this bug reported in our forums for awhile and it's nice to see quick fix out there.
One word of caution: technically this is for the Vodafone/Euro version only, though we suppose it may work on the unlocked variety if you really, really need it. If not, we bet that'll be out lickity split.