Comment trixbox & Polycom IP Series phones (Score 1) 405
You can run the server software ( Free ) on any older machine and the phones are nearly open source with the options they have.
The Phone can be expensive but any unit ( phone, cell, PC etc) that can run SIP based telephone calls will interface fine.
For example I have a HTC Dream with android and the SIPDroid app works very well.
Links:
http://www.trixbox.org/