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A regular old phone line, is called a POS line, and only uses a single pair (2 wires) to make it happen.
The Blue pair is used for the single phone line, when your using a Cat3 (3pair, or 6 wires...consisting of the Blue, Orange, Green pairs) or Cat5 (4pair, or 8 wires...consisting of the Blue, Orange, Green, Brown pairs)
Either way, the Blue pair ends up being the very CENTER wires of the plug (RJ45-Ethernet/Cat5) or RJ11 (telephone jack plug/Cat3, or lower.