I've just read The Cathedral and The Bazzar by Eric Raymond, and when I was looking at some PBX solutions I was very pleased to find Asterisk, the Open Source PBX. This is software written to run on PC class hardware to provide features commonly found in more expensive proprietary systems. (ie Lucent/Avaya Nortel et al.)
The Asterisk project is managed by Mark Spencer, who originally wrote the software. Now code is reviewed and "patched" or improved on a daily basis by "contributors from around the world". Mark Spencer has a company, Digium which manuafactures hardware to interface PC to the PSTN, either via T1, or of more interest to small business, they have products which can connect via individual POTS interfaces.
I hope I have time to put one of these together and experiment with it this winter.