PABX - Private Branch Exchange (a small telephone system with call switching capabilities - I'm not sure about the letter A in PABX).

Small PABX can be 1/2 (one external and 2 internal connection) and goes up to several external lines and lots of internal that can be analog or digital (ISDN).

The complexity also grows with the functions that are implemented in PABX in exponential fashion.

PBX 1/5 with only PULSE dialing had 90% implementation in hardware and 10% in software - 15 years ago - system configuration was only ONE jumper.

PBX 2/18 ISDN with 10 analog and 8 ISDN ports has far more comlex PCB (4 layer board), but this is only 10% of the system implementation. 90% is the system software that 80C251 can hardly handle.

System configuration is a WINDOWS based configuration software with RS232 or ISDN system connection.

INDRA, you can choose in the range described above, If you choose the latest you can count with at least 5 years of development.

I suggest for your home usage to build a simple 1/2..5 analog PBX with only basic switching functions: configurable incomming call distribution, call forwarding and call hold/retrieve. You can put your application in a 89C51.