reply to PToN
Re: VoIP system I think you can find a good system that doesn't use proprietary phones (like Avaya does). Something based on Asterisk. If you are a CLI person, raw Asterisk might do the trick. There are a number of GUI add-ons for it. FreePBX offers a distro.
If you want something more polished, Digium offers a GUI for Asterisk. It's more expensive, but looks to offer an enhanced end-user experience.
I wouldn't bother with SIP trunks- we tried for over a year to get acceptable audio quality and never could.