It's probably a 5100-series system, which is a 45 ohm talkback system, manufacturered by Valcom. The amplifiers in the 5100-series are in the head end equipment and not in the speaker. Valcom paging systems (i.e. one-way) typically require 2 pair for each station, one pair for audio, one pair for power. Valcom talkback systems ( i.e. intercom, bidirectional talkpath) utilize a single pair for audio and since the amplifier is in the headend, no power is required for the station speaker. In most 5100 intercom installations, a second pair is used at each station for a call switch which is used to initiate a call to an admin port. Expandable to 360 stations in increments of 24.
Hope this helps.
