VoIP (Voice over IP) is rather a broad term that stands for sending compressed voice over IP-based network. I believe, the question about pairs came from your consideration to use hardware VoIP phone set. Actually, it's a little computer with Ethernet interface inside, so as far as wiring is concerned, it's the same as for any other Ethernet device - they use pairs #2 and #3 from 8-position modular jack. So the answer is: they don't use more than two pairs. This is based on the assumption that power is provided by AC adapter connected to an adjacent power outlet, not delivered through the 4-pair work station cable.
Dmitri Abaimov, RCDD