It would be nice, but bandwidth isn't free, so it's one less burden on ANET if the players manage their own voice chat services. I suspect a similar excuse is used for guild wars utilising fan-forums instead of having an official one (forums have the added cost of moderators too).
If anyone has any better theories, let me know, but that's my view on the topic at the moment.
I believe the prophecies collector edition came with some teamspeak stuff (the program, a code for a months worth of server?).