Hello,
For a mini PABX in Home Use, you can try relay switching with line voltage feeding (24V), dtmf decoding with a very famaous ics (8870 or something like that) and ring detection by optoisolator. This application is suitable for 1 external line. Otherwise, for more than one external line You will need analog switch matrix, 2 to 4 wire conversion, dtmf encoding decoding, dial tone supply, music on hold etc.

I have some experience on relay switch, analog and digital PABXs. If you wish I can write more...

TAYYAR