We have been using phonebooth.net for our 4 geographically distributed work-at-home offices (Pittsburgh, Dallas, Austin and Denver), and it's worked pretty well

We have been using phonebooth.net for our 4 geographically distributed work-at-home offices (Pittsburgh, Dallas, Austin and Denver), and it's worked pretty well so far. They worked (and are working) hard with us to get out the little glitches, and we got to deal with their actual developers, since it was (still is?) a beta test situation. They don't handle faxes, though. Pretty good deal, though: $20/month/line, 5 line minimum. Cheaper than POTS, and much more flexible. (I just dragged my Polycom IP phone out to our farm and plugged it in for the first time, and I'm up and running with my phone line with no obvious issues.)