In a telephone cable, whether it is a 25 pr, 100 pr or 600 pr cable, the pairs are bundled in 25 pairs, and sub-bundled in five groups of five. There are five twists used and repeated with every five pair. When I say five twists, I mean that the number of twists per foot vary between the five. The reason for this, first is that by twisting the pairs, you contain the EM field that each pair generates to a smaller radius. By varying each pair of five in a sub-bundle you do not have any two pair next to each other with the same twist, therefore preventing them from coupling and creating a larger radius of the EM fields emitted. This cuts down on cross-talk between pairs. Reducing the radius of the emitted field is the primary reason for twisting pairs.