To refresh, the question received was:
In the context of that question I was talking not about the nature of the PBX itself, but the nature of the phone lines that are used to get the dial tone and the type of phones that will be attached to the system.
An IP PBX uses a connection to an IP network versus legacy PBX systems that can use analog trunks or ISDN lines. I was also referring to the fact that existing analog and digital phones can still be used in hybrid systems with the appropriate gateways.
Both systems are digital, as the reader describes.
Most of the legacy analog systems were replaced in the U.S. many years ago, and in the rest of the world with the Y2K upgrades -- though it is not surprising to find many areas of the world where analog systems are still in operation!
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This was first published in May 2006