Depending on the codec you are using typically you need about 64k per call for good voice quality. You also need to remember that the remote party has to have the same available, so a dialup line will not work.
I have axon installed on a windows xp home computer without the firewall. I also have a linksys sipura 3000 fxo adapter. Sometimes the linksys has problems sending calls to the pbx sometimes it works great. Has anyone installed it on windows server system and what has been the results? Thanks