Analog uses one pair for voice, it will use a second pair for data and can use a 3rd pair for OHVA (off hook voice announce) and there's a few systems that use the 4th pair (Merlin Classic comes to mind) but I don't know what it's for. [Linked Image from sundance-communications.com]
Digital can sent all the above information on a single pair. [Linked Image from sundance-communications.com]

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