Linux based VOIP PBX?

[Larry McVoy]

Hi, sorry for the somewhat off topic question, but I'd love to know if there
are any production quality Linux based voice over IP products out there.
We currently use a 3com NBX and it sort of sucks.  It works fine on a
LAN, it sucks on a WAN.  I'm open to replacing it, but if I did, I'd want
to go with a Linux based system, for lots of reasons.  If you know of such
a product, or are working on one, please contact me.  I don't want a free
product, I want to pay for it and I want it to work.  100% of the time.
And we need the voicemail/conferencing/menutree stuf that you get with 
a real product.

We have $15K into the 3com, so that gives you some idea of our willingness
to spend money.