Cheap and Reliable IP Telephony?

anomalie asks: "I am trying to sell IP telephony to my employer. The idea was shot down once already because of the cost (using a Cisco solution). I would like to find a cheap but reliable IP PBX because everyone liked the idea of IP telephony, just not the price associated with it. I need a system that could initially handle about 80 users at a single location, and eventually handle about 350 users at 7 locations. The two systems I have been looked at so far are Asterisk & Pingtel's SIPxchange IP PBX. I'm not looking here for a final solution, just some starting points for more research. Any feedback/tips/warnings from the Slashdot community?"

"I am looking to have at least the following capabilities:
-Auto attendant
-Handle a PRI (hopefully allow forwarding of old PBX DIDs)
-Handle long distance T1 (we would initially segment off some channels from our current PBX)
-Handle WAN Traffic so we could utilize our unused channels for long distance from other locations
-Forwarding of voicemails to email

Nice optional features:
-Web based GUI for voicemail administration
-GUI call manager

Eventually, we would have relay units at the other locations to handle the local calls and call routing and have 1 central PBX at corporate headquarters."