Do you need a PBX or a star system? That is, are your phone lines going to be multiple POTS lines or one or more T1-type lines?

Edit: Also, personally, I'd be more inclined to go with a phone company's product, like Nortel, Lucent, Siemens/Rolm, etc., rather than with someone like 3Com. Those other companies have a solid track record for near 100% uptime. 3Com, etc. are based in the computer industry, which doesn't have as solid a track record in that area. Of course, you're likely to pay more for a real telecom product. I've often purchased used stuff that works just fine.

And, personally, I've never used a phone system that was as good, from an end-user point-of-view, than the Rolm phones. Never administered one, though, so I don't know how difficult it would be, but I can't imagine it would be any more difficult than any other.

Plus, any of them will be fairly easily networkable. All the telecom ones will work off of a dumb terminal, so just attach that to a serial port on a networked computer and ssh/telnet into it for access.