Over the past week, I have been designing my own PBX, with capability for 16
lines and 4 trunks. I looked at all design options I had available to me for
switching, voice transmission, etc., and the best I have found so far is a
line of ICs by Mitel -- Their SLICs, COICs and analog switch matrix devices.
With very few additional components, you could easily build a PBX.

Have a look at http://www.mitelsemi.com

Also, Teltone's (http://www.teltone.com) dial pulse counter ICs and progress
tone generator ICs are helpful.

Using the Mitel 'system', you can construct a PBX with no transformers or
capacitors in the audio path -- Just connect the junctor on the SLIC to the
analog switching matrix. Connect the right matrix points together and you
have a connected call.

- Keelan Lightfoot