A major difference between PABXs and Public Switches is scalability. A Telco Class Switch (5ESS like) can be expanded up to 250.000 local circuits, 75.000 tandem circuits or a mix of them. Also can handle about 2.5 Millions BHCalls, with the CPU expanded at it`s limits.
More than 30.000 Erlangs can be managed, with the last generation circuit switched exchanges. Many other services can be provided concurrently, like ISDN, VoIP, IN, etc. Billing capabilities are awesome.