The original APO system was a key based phone system.  A key based phone system has multiple outside lines going to the phones.  The following figure is a phone from an old system showing the largish cable that has all the phone lines in it and the mechanical switches for selecting a phone line.

The modern IP PBX systems use a network cable and data to handle phone calls from SIP phones.  IP PBX means a PBX system using the internet to manage phone calls.  The difference between the old parallel phone lines and switches is now software and a network.  There is no concept of individual lines to a phone.  The PBX is now based on the idea of a call between two points, and the call can be parked and picked up by another point.  When the call is picked up, the data flows.  The outside lines can be the old lines converted to data streams, and outside lines can also be internet SIP trunks that have 1 or more data streams attached to it.  At the SIP provider, phone numbers can be purchased and attached to the trunk, too.  When a call comes into a number on the trunk, the available data path is used to connect the call to the PBX.  If there are no data paths, then the called number is not available.  The call is transferred to a fallback number if available.  For APO, there are 8 lines in, which is 8 data paths.  A possible configuration is a number of local lines, and then a few long distance lines on a SIP trunk.

The new IP PBX is the current state of the art for PBX systems.  The buzz words are SIP, SIP phones, SIP trunks for telephone lines, and  IP PBX systems based on Asterisk, an open source project supported by SIP telecom companies. 

The advantage of an IP PBX system based on Asterisk and SIP is:

IP PBX - freepbx and Asterisk