Non-gigabit Ethernet over twisted pair cables with RJ-45 connectors (either 10BaseT or 100BaseTX) uses pairs 1, 2, 3, and 6, whereas gigabit Ethernet (1000BaseT) always uses all 8 pairs. Some PoE switches transmit the power using pairs 4, 5, 7, and 8, whereas other PoE switches transmit power using pairs 1, 2, 3, and 6. The 802.3af PoE standard requires devices to support both modes. I always wire all 8 pairs whenever I do Ethernet cabling, since we need to connect these cables to a PoE or Gigabit Ethernet port.
can you have a gig VoIP phone work on just the 1,2,3 & 6
No, gigabit ethernet use all eight wires for data. Use ethernet switch to connect two or more gigabit ethernet devices. Or use just fast ethernet phone - it's cheaper and you can split cable (although it's not recommended practice).
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