I have been looking at
www.digium.com they are the main developers of an open source PBX which runs on Linux and Windows called Asterisk. From what I know it looks like it could be useful to you.
The neccesary PC cards are readily available, basic ones go on ebay for sub £20 brand new.
Depending where you are there are consultants around who will set it all up or as it is open source, buy some bits, download the programme and set yourself up.
Memetic