It's "worked" from the little testing I've done. TBH, the VoIP on any game I've ever played was always close unintelligble and usually annoying if it was coherent. Usually it's too low bit rate and too quiet for me to understand well, no matter what I set the VoIP at. (Talking about UT2k4[and UTXMP], CS, CS:S etc...)
One thing important to note is that VoIP is a configurable server setting. Perhaps most(all?) of the servers have it disabled. I don't have a copy handy but maybe the dedicated server download has VoIP disabled in the .ini's. In the end, FMI has about 0 control over that, you gotta talk to server admins. I don't see any server admins weighing in on this also. I also wouldn't be the least bit suprised if Dubious turned it off in the ini's he put together for the dedicated server install to make server loads lower (and therefore make UTXMP look better.)
Anyways, noone's shown anything to make a good case it doesn't work. I hear this: it either doesn't work period, doesn't work correctly on their system as a result of UT2k4, doesn't work properly as a result of UTXMP or it's disabled on the server(s.)
Can you find a 2k4 server with VoIP enabled and does it work there? If yes, have you talked to admins of whatever UTXMP server you're playing frequently and asked if it's enabled?
BTW, If I sound nasty in this post, I apologize for the tone. I'm trying to be helpful not nasty.