So if your using the 4 wires used for ethernet, Standard wiring diagrams make this the Orange, White Orange, Green, White Green cables, which are 1,2,3 and 6
You mentioned using pins 6 and 7, did you mean 7 and 8? because obviously using pin 6 for both is not going to work. But you "Should" not experience any interference/Crosstalk if your also utilizing the blue/brown cable pairs for voice, I've done this in the past but now with gigabit networking, Those "Extra" pairs of wires are not utilized for the additional throughput so just be aware that your stuck at 100 base T speeds.